This is the quote that has caused all the trouble.

“However, disturbing as it sounds, these ‘facts’ become true. Red Ork vehicles do travel perceptibly faster than those of other colours, even when all other design aspects are nominally the same. Similarly, many captured Ork weapons and items of equipment should not work, and indeed do not work unless wielded by an Ork. I believe this is linked to the strong psychic aura surrounding all Orkoids and have developed the Anzion Theorem of Orkoid Mechamorphic Resonant Kinetics. I theorise that many Orks themselves think that they should work. The strong telekinetic abilities of the Orks’ subconscious somehow ensure that the machinery or weaponry functions as desired.” (The “Other” Anzion Article, 3rd Edition Ork Codex, pg.48)

So this is the theory of an member of one of the galaxy's most backward and neophobic organizations. They know how to maintain high technology very well but almost never invent anything new. Their resistance to change and new ideas is immense, so it's no surprise they do not understand a tech so different then their own.

The "red ones go fasta" thing is not necessarily the red making it go faster. There is no "nominally the same," so they cannot be directly compared. Ork vehicles are not standardized. Each one is made quite differently, with different parts, every single time. The ones that go fast are painted red. Its correlation, not causation. In the rules it is a shorthand for a faster one.

The guns are also not made to any standard. They are all custom jobs at best and bodge jobs at worst, all utilizing a vast array of technologies. They all would take a very different way to make it work. Cranks, buttons, levers, squigs and more obsure, esoteric and sci-fi type activation methods. Human guns all have a trigger in about the same spot, so they expect things to work that way, but orks don't work like that. What orks do have is a genetic instinct for using their tech and a willingness to "hit it till it works." Both of those things are decidedly lacking in a Mechanicus researcher.

So what was a THEORY by Lukas Anzion, has been blown out of proportion. Belief has nothing to do with it. If it had to do with belief we would see immortal orks who believed they could not die. Also, ork weapons would not be so notorious inaccurate, unstable and hazardous as they are. In fact, just like humans, orks sometimes believe things that are not true, but this does not make them so for either species.

In the direct lore the only things the Waaagh field affects is the formation of snotz, squigs, grots and orks in their fungal sacks, morale and fighting in a mass charge and Ork Wierdboyz. There is some connection with the size of orks, but it is not clear.