GalagaGalaxian said:
The logic is, it gives their established setting and point in time Tyranids for the players to fight. I hope they wake some Necrons up early too. Because I want to have Necrons in Deathwatch. Besides, GW seems fine with FFG's work, though GW has never really cared too much about rigid canon. Everything with GW's seal of approval is cannon, equal in their eyes, from their own stuff, to deathwatch to those crappy Goto novels. Whenever established canon conflicts, choose what works best for you.
Actually GW regards the rpg as ... hazy canonically iirc. I shall try to find the quote.
Necrons are already active in localised regions, according to the Necron codex, in 815.