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.
Once again, have you even read ANY of the heresy novels on the subject? Thats, like, nothing at all like what happened, even to Magnus' own admission.

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.
That just goes contrary to how development works in so many ways its not even funny. In order to do any of that, its not a case of a 'super human brain' coming up with anything; you need to continue research into the fundamentals (metallurgy, ballistics, computing, etc), set up a production line, set up a supply line for said production line, then set up a logistics chain across a distance so enormous you need to step into hell just to get anywhere. Im not entirely sure how you expect any of that to happen when every chapter is constantly undermanned due to endless fighting and attrition, when even chapter survival isnt a given and when everything is handled by the AdMech. Once again, its like if you are talking about this setting you imagined, not whats actually writen anywhere.