T'au aren't much better than the Imperium, they're just better at hiding it behind good PR. At the end of the day, they might not be actively xenocidal, but you never see an Auxillary end up in a position of real influence in the T'au Empire. And there's a lot of circumstantial evidence that the entire thing is backed up by at least some degree of mental influence like the whole bit where they give a "Translator Device" and all of a sudden an extremely hostile insectoid race is perfectly willing to join with the Greater Good. (That also has something to do with their plot armor, like that entire story that takes place where every type of Imperial Assassin takes a swipe at an Ethereal backing an invasion of an Imperial world, and every single one of them fails due to some degree of Suspicious Coincidence--like "Instincts" of a bodyguard somehow detecting a Vindicare lining up a shot and zeroing in on them before they can take the shot. And yet the Culexis just walks in, and while everyone feels weirdly apprehensive when it's nearby, nobody notices them despite them identifying things that were much more subtle before the Ethereal was in any danger, they murder the Ethereal without issue, and then leave.)
Farsight clearly figured something the fuck out when they got isolated from the Ethereals for a while, after all.