Funny how you keep blatantly ignoring what the Lexicanum says:
Searching for a solution among the Librarium of the Ravenspire, Corax found the ancient books of genetic research providing the techniques used to create the first enhanced warriors of the Emperor, those techniques Corax duplicated to create Spaces Marines at a frightening rate.
Corax created his super marines with the tecniques used for the first enanched warriors of the Emperor. Which would be either Thunder Warriors or Adeptus Custodes. Not the primarchs.
There's a key diference between the lexicanum not saying something, and saying something that contradicts an obscure author.