>why should he listen to literally the dumbest primarch?
Lorgar was flawed as all Primarchs are flawed, but "dumbest" is hardly accurate unless you're one of the faggots who thinks religion is dumb in itself.
>Dude couldn't take calth, even with his daemon friends.
But he made the statement he needed to make, and drove a gigantic scar into the Ultramarines psyche. It is also worth mentioning that Calth was going awesome until Kor Phaeron disobeyed Lorgars orders and attempted to corrupt RG instead of just killing him when he had the chance, which was the tipping point of the battle that allowed the Ultramarines to come back and win the day.
they also had to lose, because plot
>The word bearers have also been completely irrelevant after that
Only if you skip over any fluff that isn't Black Legion. They're not at the level of the Black Legion, but they're still extremely active in 40K, and are second in organization and strength to the Black Legion (although maybe Red Corsairs have surpassed them now).
> while the Ultramarines remain the GREATEST OF THEM ALL despite ten thousand years of stagnation.
A. They aren't stagnated. The Ultramarines, while still being extremely rigid and hidebound, still adapt to new tactics and technology as it becomes available to them, al la Centurion Armor and Hunters/Stalkers and Tyrannic War Veterans and their tactics.
B. They're the flagship line of the 40K modelling scene. Of course they're going to win, as plot demands they can't permanently lose anything.
>Quite obviously the Codex Astartes is a greater truth than the primordial "truth".
A. Guilliman himself believed that the Codex was a rough guideline and far from perfect, and needed to be adapted to as needed. It wasn't a "truth" to him moreso as a mindset.
B. We're talking about Pre-Fall Lorgar here, and the relevance of faith and the divinity of the Emperor and Guillimans refusal to give those ideas credibility.