The emprah declared it. You dare question the emprah's words?
This is that Grace you speak of seems well on her own way to heresy as far as the emprah is concerned.
Liked when Magnus tried to warn daddy of the incoming rebellion, the emprah didn't care about said priceless intel given for almost free. The emprah simply ordered the thousand sons to be curbstomped.
That is false. There's plenty of spech merines quotes addressing the emprah as their god. They believe that when they die the emprah will guide their souls and they'll get to sit at his right side in the afterlife. And they follow the dogmas the emprah left with such fervor to make the most devout eclesiarchs weep with envy.
Or what, mister posthuman couldn't develop a significantly better wargear or vehicle chassis after 10 000 years? It's like claiming our modern commandos fight clad in the finest raw fur coats and master crafted wooden spears. But the emprah ordered them to use that design of power armor and bolter, and the spuch merines obeyed with no question for ten millenia when they could've used their posthuman brains to develop something-anything multiple levels above.