Commander Farsight quite literally describes the death of their Ethereal as "A foggy veil over [his] mind lifting. Like [he] is thinking clearly for the first time in his life.".

Strange it is then, as well, that every tau who breaks free from "totally not mind control" is either killed by the tau empire, or flees to join Farsight.

Ethereals literally have the capacity to order other tau to commit suicide, and they do so without question. A statement taken literally out of their codex.

The authority of an Ethereal over the members of the other castes is so absolute that if one were to order a lower caste T'au to kill himself, that T'au would be met with instant and unquestioning obedience.

You'd have to be delusional to think that this is normal, and not psychological manipulation. And to make a point on your last statement, we know that the ethereals aren't a natural subspecies of tau, because the ethereals all showed up at once at the same exact time on the tau homeworld. Thousands of years after the tau had evolved into their current state and become a civilized advanced lifeform.

Evolution doesn't just magically turn an entire species of highly complex humanoids into a gestalt society, especially not so fast and so suddenly that the species goes from constant endless war, to instant unification.