Can't remember the exact source, or if i even read it, it may have been something someone told me. I was posing it towards Shaten more than anyone else to see if it was the same thing. For all i know, i may be confusing another series entirely, i read far too much.
But as i said, either wat what it says, or where it comes from is not the point. The Point is more towards what constitutes Canon in the 40k universe? Technicly, if i wanted to, i could create a small nebula off in some corner of the Galaxy where Tau, Eldar and Humans live together in harmony, and because of the way the universe is designed, there is no one who could dispute its validity, not even GW, unless they wanted to do some serious rewriting.
However, when talking about key features, such as major players and critical events, we have to assume that things published by GW have their historical seal of aproval.