Board Thread:Warhammer 40k Novels/@comment-27376167-20151210113237/@comment-4582688-20160414011423

The Tau were still primitives at the time of the Great Crusade, so that wouldn't work.

Another bit of information pertaining to the Lost Legions is found in the background information for the Deimos Whirlwind Scorpius: that it was developed and used during the first inter-legion civil war. No other information about this conflict is ever stated again that I know of. The Scorpius was used widely during the Great Crusade so it wasn't talking about the Heresy.