For a very long period of time, there was a prevailing fan theory that the Ethereals weren't just psykers, but were psykers created as the result of genetic meddling by the Eldar of the Craftworlds, in a kind of history-rhymes version of the space elves's own creation myth as servants of the Old Ones. However... they're not. Tau'faan barely have a presence in the Warp, and this includes the Ethereals; they have been subject to rigorous testing in that regard... by the Drukhari witch cults, who instead found that the actual method of control was great for making into combat drugs. I'll spare you the details. =]

But yeah, the modern canon leans heavily on the idea of the Ethereals actually exercising control via the medium of some kind of alien pheromone, which from this old fan's perspective is something of a cop-out. It feels to me like a ham-fisted way to try and deal with the unbelievable whining about the T'au's existence as "not fitting the established setting" that started from their introduction in Two Thousand And No I'm Still Young Shut Your Whole Entire Up, which -- as someone who was there even on the periphery of it -- was as loud and intolerable as... well, as everything else screamy, loud and intolerable in the Warhammer 40,000 fandom.

Speaking solely for myself: I think the T'au absolutely do fit the setting both as they are and as they were. They're actually one of the last bits of genuine political satire in the game, with all their NATOspeak weapon designations and bloodless naming conventions about warfare being broad satires of the neoliberal order prosecuting an illegal war in Afghanistan and later Iraq. It's not a grenade launcher, for example, it's an Airburst Fragmentation Projector. More centrally to my thesis, however, they're also a powerful contrast within the grim darkness of the far future, because they were (at the time of their inception) extremely naive about the nature of the universe of which they were a part and acted like capital-S Science! could fix everything. That was the central irony; they were symbolic of the wondrous technology and fundamental hope for the future that fascist humanity had spent ten thousand years stamping out wherever it could, only survivng through their own unbelievably rapid technological development and sheer blind luck. Making the Ethereal caste pheromone-controlling versions of Fu Manchu and cranking up the re-education camps idea is both a little too rooted in Sinophobia and "yellow peril" ideas for my tastes and also, in my opinion, was something that was both unnecessary and clashed on a fundamental level with the satire they were trying to make. It undercuts the fundamental naivete of a faction if their leaders are secretly a bunch of scheming egomaniacs twisting society for their own ends.

At least, that's my take as a long-term fan of, and there's really no getting around it at this point, twenty goddamn years Jesus Christ help I have fallen and I cannot get up

=]

Phil Kelly is pretty much the only game in town right now when it comes to the creation and exploration of T'au lore and stories. However, he seems to delight in retconning stuff that had already been established for no reason (it seems) other than "he didn't know about it or forgot it existed". The "T'au blood is red" thing was a meme for a bit precisely because of the Farsight books. Not my favourite. But then, I've not read the Farsight series in a while, so Phobos is one up on me there. =]