But who cares? This is the grim darkness of the far future, right? The Imperium is a totalitarian government that isn't exactly known for its freedom of thought and expression. Even the existence of Chaos is kept from the majority of citizens because it is information too dangerous for them to know. Everyone in the 40k Universe has their own agenda, and they are not above propaganda to further their goals.
Just because someone like Mat Ward has written the latest edition of the Codex including details we despise doesn't mean that's how it's going to be forever. When the current codex/fluff authors are eventually replaced and new editions of rulebooks and codices are written, things will undoubtedly be retconned again, perhaps in a way that players will actually like. I just see the current version of the fluff as merely the latest propaganda distributed by the powers that be, and as such, subject to change. So there's no reason to get too upset over it.
I'll end with a quote from Ward himself:
"The history of the Imperium since the Heresy is not a continuous story. There have been periods of rebellion and anarchy; times when the balance of power has suddenly changed and history has been quite literally re-written."
--p9, Codex: Space Marines, Matthew Ward, 2008.
History will be re-written again.