Board Thread:Warhammer 40k General Discussion/@comment-26056880-20150130004022/@comment-24037808-20150203211142

If forced to find a solution, i could say that "mortal wounds" are reffered to..mortals, a normal human being, or even an enhanced one (space marine).

Probably to emphatize the unique traits of the Primarchs, a mortal wound (for any human) could still not be the end for a Primarch as well as  (if not more) for the Emperor.