Dawn of War is never stated to be entirely part of continuity, ever.
Official studio fluff, Black Library, Forgeworld, and Fantasy Flight Games.
Those are the four companies (Three of which licensed by GW) that can create canon IP, according to Aaron Dembski-Bowden.
Notice that Relic is nowhere in there.
Although some aspects of Dawn of War, such as the Blood Ravens chapter, have been integrated into official fluff, that does not imply the entire thing is canon.
That said.
It's not impossible for a chainsword to harm a Carnifex, honestly.
It's just not likely, and will take more than one swing.