The description of the Gith, particularly the Githyanki portrays them as them deadly, implacable enemies of Mind Flayers, so much so that they took down the unbelievably vast and powerful ancient Illithid empire, and all but exterminated the brain-eaters.
Yet looking at their game stats in the Monster Manual reveals no major advantage the Githyanki possess. The average Githyanki is a CR 3, while their elite knights are CR 8. The average Mind Flayer is a CR 7, but has some pretty effective psionic powers compared to the Gith, who don't seem to have any particular resistance vs them. If you add in the variations and powers revealed in Volo's Guide, the Mind Flayers are even more formidable as a group.
Is there a description of the tactics the Gith employ against Mind Flayers that would allow them to all but exterminate them? They don't seem that mismatched, but the impression one gets from the text is that the Gith are the scourge of the Illithid.
I'm looking for either a game or game fiction source that describes how one race can so dominate the other despite only a tiny variance in stats. I would accept answers based on actual play experience, particularly if they point out some major factor I am missing.