@sumbozo
As someone who doesn’t play Warhammer 40,000, but has looked at things related to it on the Internet long enough to get at least a little familiarity to the lore, I can confirm this.
Oh yeah, the betrayal also causes a civil war that severely changes the Imperium, mortally wounding the Emperor, killing a few Primarchs, and other things that turn the Imperium into the super-paranoid and ultraconservative anti-heroes they are in the present.