[@Jbcool] Well, the Tau were originally discovered in the 35th Millenium, at the time possessing Stone Age levels of tech barely above the discovery of fire, and would have been killed ASAP if they hadn't been obscured by a Warp Storm for thousands of years. A Warp Storm that might not come about for thousands of years, or if it's directly prevented by a sufficiently strong entity...

The planet of T'au is located in the Eastern Fringe. If it so happens that the Imperium comes across it earlier than it otherwise would have, the native xenos have only themselves to blame for not being smart enough to comprehend the Imperial Truth. Ahemhemehemhem.