I've only read that from online discussions but I don't think I've stumbled across it in any of the codices. I'll have to take another look through the books. I think it could have come from that one old book that was meant to be an Imperial scientists observations on Xenos that he gleaned from his own research and autopsies. Never did pick up a copy.
The original lore had the Ethereals appearing in one single night, following a series of strange lights being seen in the air at night. They took control of Tau society instantly as Tau instinctively followed them. There was also an autopsy where they discovered the difference between ethereals and other Tau was the presence of a gland in the brain that came from another animal from a planet a long way away, which couldn't possibly have naturally evolved.
There was other stuff as well - it was never directly spelt out, but it seemed to be clear that the Tau were artificially supported in some way, and the Eldar fit better than anyone else.
(Remember,. their entire society in the original lore appeared basically overnight. They went from medieval to space flight faster than thought possible).