I have been flicking through the C:SM again, and cannot feel that (in the vein of the Tau Empire Codex), it was written as pure Ultramarine propaganda to appease the population of Ultramar and make them feel nice and snuggly when they go to bed at night.

I mean, seriously, Calgar defeating an Avatar of Khaine barehanded? Was the Avatar already dead and Calgar just beatdown on the carcass for fun? Aren't the Avatars all fiery and stuff? But Calgar didn't get burnt?

I don't mind some slight exaggeration in a codex, but I don't think you need to make armies sound so incredibly awesome with stuff like this just to make their players feel better about their choice of force.