I would rather the vagueness and mythological status of the primarchs as being true demigods akin to ancient legends and such today.
If you think about it,in current 40k(the age of the Dark Imperium) the average Imperial citizen is mostly in the dark when it comes to what really happened during the Horus Heresy.
Primarchs are true demigods to the imperial citizens,embodiments of either virtue and sacrifice(Sanguinius - Loyalist) or depravity and vanity(Fulgrim - Traitor).
Hence,they are indeed shrowded in mythology,representatives of a terrible age of destruction and where legends were born.
Yeah, people tend to forget that what the reader knows literally
nobody is aware of in-universe, save for, I don't know, a literal handful of characters. Knowing like anything about Angron, even
his name, would turn any standard Imperial citizen into a nice crisped peace of meat in a heartbeat. You can easily skip all HH prose and just ignore it if you want to enjoy the world as is.