I feel like the part of it being a mix between Molag and Hircine is completely a wild hunch by the writer of the book, as its theorized as such because of their immunity to sun and favor of the moonlight (due to the old dark stalker decreasing sneak time only during night) The new Vampire changes goes against that idea because the new dark stalker decreases it in general (which to be fair, is purely QoL changes) Does this retcon the book? in a sense. It makes the theory even more outlandish because we now have no favor to night, just immunity to the sun.

Overall I wouldnt take the text seriously, it was obviously written in the eyes of a speculatory researcher who does not know how ESO vampirism was concieved and is just providing theories based on (previous) effects the strain had, where as the player character obviously does