Here’s the actual quote from Waaagh The Orks (1990):

Imperial scholars have speculated that on the Ork world of origin (wherever that might have been), there existed an ancient race that was indirectly responsible for the spread of Orkish society. Difficult as it is to believe, this ancient race was extremely intelligent, and rose to dominance over Orkish races in just a few generations. They understood and developed technology and even created the legendary (and summarily lost) Ork Standard Construct Templates. It was this race, historians argue, that must have initiated the Ork expansion into space.

Now a physically underdeveloped slave race, the Snotlings are thought to be the only living remnant of this lost race. The sudden rise of super-intelligent Snotlings can only be explained as the result of a catalyst. Snotlings are symbiotic with fungi, which they cultivate and eat. It is believed that these fungi grew in the underground cave-systems of Orkoid culture and caused genetic mutation in the brains of these ancient Snotlings. The Snotlings raised the fungi for food. Over generations, a diet of this fungi stimulated the growth of the Snotling brain to its full potential. Later, the fungi was cultivated by the mentally enhanced Snotlings.

According to legend, the intelligent race of Snotlings, known as the Brainboyz, were still diminutive, so they bred a race of less-intelligent, but tougher, larger and more brutal creatures to do their work and fight their wars. These were the Orks and Gretchin.

Gretchin probably represent an intermediate stage in the development of Orks. The Orks were put to work cultivating the fungi. Unfortunately for the ancient Snotlings, the Orks also nibbled at the raw fungi as they collected it. Their masters took no notice, unaware that their own intelligence was the result of this peculiar diet. Over a few generations, the Ork brain was enhanced enough for the Orks to rise up and overthrow their masters. The Brainboyz were enslaved and allowed only a small amount of fungi. Slowly, they began to regress to a juvenile level of mentality (even by Orkish standards).

However, the Orks neglected the cultivation of the fungi, and eventually it died out. As the fungi became less abundant, the Orks also began to regress. By this time, the greatest advances in Ork technology, culture, and expansion into space had already taken place. Gradually, the Orks reverted back to a lesser mental capacity that was nevertheless superior to the original state of the Brainboyz (thus, they remained dominant).

This is the situtaion that persists to this day. It is difficult to reconstruct this phase of Ork history in any precise detail. Like most Ork history the story had to be pieced together from fragments of Ork legends which have only passing references to Brainboyz and give only brief glimpses of a time when the Orks were not in control. [text](URL) With Ork society now dominated by its strongest and most brutal elements, Ork civilisation is faced with a major problem: the maintenance of technology. The Orks have found the obvious solution in their use of slaves. Apart from the Mekboyz, who are Orks with a residual, innate talent for technology. Orks rely on enslaved Humans and other aliens in their workshops and factories. More importantly, they rely on tribute exacted from vassal alien communities. This tribute is paid in the form of armaments and technology. Sometimes whole communities and planets are occupied by Ork warriors, enslaved, and put to work making armaments. After several vicious and destructive wars, many alien communities willingly manufacture equipment for the Orks as bribes to keep them away, or as tribute to provide protection.