Space marines in-universe function EXACTLY the way they do on the tabletop.
Most of the things that they are said to be capable of, and most of the story of the primarchs, is propaganda, the same as the tales of the military prowess of the spartans or persian immortals or samurai. Historically speaking, if you think of a particular group of warriors as especially fearsome, odds are extremely good that that is more due to how well that group was able to propagandize their exploits than anything else.
The Primaris marines and Guilliman are part of a new technological wave from an influential, heretical faction of techpriests using the pre-existing legends about Guilliman as ideological backing for their power grab.
The emperor was a human, or perhaps several humans. whose actions during the DAoT were naturally translated into legends, as is all history. He was as real as Heracles, Jesus, George Washington or Agamemnon - a real human with actions embellished, misattributed, forgotten and added to create a character most likely entirely unrecognizable were you to actually meet him at the time. Given how fast this happens in our current era - a couple hundred years - the thought that the actual thoughts, actions and persona of the emperor could be in any way preserved through 20,000 years is beyond comically naive. Certainly there is a corpse on the throne, and a warp entity that appears to correspond to peoples' image of what the emperor is and means to them, and people believe that they are one and the same.
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