Prophesies should never be taken as literal.
Tau have small signature, their society is built around controlling emotions, and the Ethereals have a strong calming effect on them. Ethereals themselves are noted for being very hard to corrupt, up there with grey knights and harlequins, I suppose. In Fire Warrior a Chaos dude notes that he couldn't get a single emotion to grab a hold on when probing the Ethereal's mind, it was like a calm sea.
But a bolt of change or daemonic possession never fails, no matter how pure and cool you are. The flesh is weak, even if the mind and spirit are strong.
Rank and file Tau are in danger of Chaos. In Fire Warrior Kais was picked by the BBEG the moment he set his hoof on the planet, and he was guided along his adventures by a voice and an invisible force pushing him on. In the end he even lost his mind. In some other bit a battlesuit pilot notes strange sensations while fighting Chaos troops, even inside his sealed armour.
Rank and file Tau are the same dudes who just under 3000 years ago were about to kill each other off. And before the 3rd sphere expansion the fire caste was ready to rally to Farsight, if it wasn't for Shadowsun and a massive propaganda campaign.
Farsight would be far more true to the Tau psychology (at least fire caste one) than the society run by the Ethereals.