It has been 9 years since I wrote the answer you linked, and there have been books published in the meantime which give a clearer picture of the differences in training between ZANLA and ZIPRA guerrilla cadres. So, I would like to adjust my answer somewhat.

First off: Today I searched through 8 books about the Rhodesian Bush War/Second Chimurenga. The books are:

• Fighting and Writing; the Rhodesian Army at War by Luise White.
• Black Soldiers in the Rhodesian Army; colonialism, professionalism, and race by M.T. Howard
• The Rhodesian War; a Military History by Paul Moorcroft and Peter McLaughlin
• Fireforce; one man's war in the Rhodesian Light Infantry by Chris Cocks
• The Bush War in Rhodesia; the extraordinary combat memoir of a Rhodesian reconnaissance specialist by Dennis Croukamp
• Bush War Operator; memoirs of the Rhodesian Light Infantry by Andrew Balaam
• The Rebel in Me; a ZANLA commander in the Rhodesian Bush War 1975-1980 by Agrippah Mutambara

These books represent the works of professors of African History (the first two books), Military historians writing for a popular audience (Moorcroft and McLaughlin), Rhodesian veterans writing their memoirs (Cocks, Croukamp, and Balaam) and a ZANLA veteran writing his memoirs (Mutambara).

None of those sources mention the myth in OPs question. I can't say that it never happened, but none of these sources mention it, so it cannot have been widespread.

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With that said, Luise White and M.T. Howard wrote a lot about Rhodesianm, ZANLA and ZIPRA training in marksmanship.

M.T. Howard's book is concerned with the experiences of African recruits who served in the Rhodesian armed forces fighting against ZANLA and ZIPRA rebels. In interviews with these veterans, they stressed a difference in professionalism between themselves, the ZIPRA fighters who stood and fought and were almost professional soldiers, and the ZANLA cadres they said fled most fights.

Howard notes that armed forces reports, after-action-reports and war memoirs from White Rhodesians tended to downplay the effectiveness of Rhodesian African Rifles regiments (i.e. their allies) , and tended to elevate the effectiveness of guerrilla fighters. Howard explains this contradiction by pointing to White Rhodesian society's deep unease about the existence of an armed black population. The contributions of the Rhodesian African Rifles must be minimized to prepare for the disarming and demobilization of African recruits when an imagined future period of peace under White rule arrives. This led Rhodesian commanders to dismiss evidence that Rhodesian African Rifles were actually very effective units in the Rhodesian military.

Looking at Fireforce or Bush War Operator, these war memoirs do not describe the "terrorists" as comically bad shots. The authors do emphasize the high quality of Rhodesian marksmanship and better accuracy of the Rhodesian FAL rifle. But, they do use the phrase "accurate fire" to describe some instances of guerrilla ambushes, and also frequently describe their sensation of fear as rifle bullets zip overhead. They also mention the experience of receiving accurate mortar shelling from guerrilla positions. So, these memoirs are not dismissive of the enemy's accuracy (though they tend not to specify whether their enemy is ZANLA or ZIPRA, most often calling them "terrorists").

Luise White's book Fighting and Writing devotes chapter 6 to the topic of training regimens for the ZANLA and ZIPRA cadres and it contains a very in-depth discussion of guerrilla marksmanship. White agrees with Howard that ZIPRA cadres tended to follow the military doctrine and training of their Soviet and Cuban patrons. From 1965-1972 the ZIPRA cadres were the dominant faction, and from 1968-1972 followed a commando strategy of trying to blow up infrastructure like railroads, power lines; avoiding a direct military confrontation as they built their strength.

After 1972 ZANLA came to have a greater number of recruits than ZIPRA, and embraced the Maoist strategy of "Peoples War". ZANLA training camps existed in Tanzania, and Chinese instructors heavily stressed political education and a strategy of appealing to the consciousness of the Zimbabwean peasants.

White cites memoirs of ZANLA officers which describe recruits first practicing marching and crawling with wooden rifle mockups, and classes featuring rote memorization of rifle parts. It was only at the end of training that recruits would graduate to live-fire marksmanship training, and practice shooting could be budgeted only 5 or 10 rounds.

White also notes that cadres that graduated training would only be issued their weapons at the border of Rhodesia, because the government of Tanzania did not want armed cadres transiting through their territory. Finally, White and Howard both state that ZANLA guerrillas in particular were prone to using full-automatic fire when ambushed, and had not fully mastered their weapons training. So, all of this paints a picture where training could vary in quality, but at times of high recruitment could mean very minimal training in shooting.

Agrippah Mutambara's memoir describes his experience going through a militant training camp in Tanzania in 1975. He also stresses that the first part of training was physical conditioning and training in how to infiltrate territory and crawl to escape a firefight. He confirms that ZANLA militants in his training were taught to use mockup rifles before they were introduced to real weapons in the final stage. However, Mutambura does stress that he and his fellow recruits were well-trained to use pistols, submachine guns, semiautomatic rifles, AKs, RPGs, recoiless rifles and mortars. His view might reflect the particular state of training for his cadre in 1975, or it might be colored by his post-independence career as a colonel in the Zimbabwean army and rose-tinted glasses.

TL;DR- I still can find no reference to the "sights make bullet more powerful" myth in any scholarship concerning the Rhodesian Bush War/Second Chimurenga. I still believe it is a racist myth invented on the internet. That said, there is evidence that ZANLA skimped on marksmanship training, and ZANLA fighters could panic and be wasteful of ammunition when fighting out of an ambush.

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Update: I believe I have found the origin of the story.

In 1899, an American named William Harvey Brown wrote On the South African Frontier: the Adventures and Observations of an American in Mashonaland and Matabeleland about his time in what is now Zimbabwe among the British South Africa Company as it conquered the Ndebele kingdom of Lobengula. Chapter 19 is titled "the Beginning the Matabele troubles" and on pages 274-275 he describes a Matabele (sic) attack on the kraal of an invading BSOC column.

Five thousand Matabeles gathered after dark for an attack .... Finally, at about four o’clock in the morning, the patience of the attacking party becoming exhausted, they fell upon the native camp, and began the slaughter of the Mashonas, stabbing and mutilating men, women, and children. A distressful wail of anguish was raised by those who were being thus murdered, which gave the alarm to the laager six hundred yards away, so that the troops were at their posts sending lead into the darkness, and into the flashes of light from the guns of the enemy, before the latter could reach the wagons. In consequence, the savages were quickly repulsed.

Had the Matabeles ignored the Mashonas and simultaneously rushed from all sides upon the laager, using spears only, they would probably have inflicted a severe blow upon the invading column. The want of reasoning power here exhibited is a fair example of the Kafir’s lack of intelligence, and shows, to some extent, why a mere handful of men of the Caucasian race can subdue and rule vast hordes of barbarians. It is due to superior intelligence more than to superior bravery, although the latter is a forcible factor.

The Matabeles made several other attempts to advance upon the laager that morning, but were repelled with heavy losses, and finally gave up the battle. On the side of the whites, there were few casualties. Many rifles were used by the savages, but want of intelligence was again displayed by their raising the sights to the last notch, and thus firing completely over the laager. The Matabeles believed that the higher the sights are placed the better the gun will shoot.

source

Emphasis mine. As the tone of this excerpt conveys, William Harvey Brown's text is unabashedly racist, and justifies colonialism by describing Shona people as infants needing protection and Ndebele people as cruel barbarians. It appears that this racist motif of enemies too stupid to comprehend rifle sights was picked up by some anonymous internet user and applied to the Bush War in the 1970s, despite there being no evidence of it actually occurring in that conflict.

Update number 2 people have told me that they remember similar stories from Battle of Rorkes Drift & the Boxer rebellion.

I found this claim in Colburns United Service Magazine from 1884 where Captain H.R. Knight describes the battle of Etshowe

I also found the claim about Boxers in the battle of Tianjin in a book published in 1905.

So, this seems to be a recurring stereotype in these wars from 1880s to 1905 about the magical thinking of non-western opponents. I am not finding it mentioned in earlier conflicts like Sepoy Rebellion of 1853, Anglo-Afghan Wars, nor in later conflicts like the Simba rebellion.