How is he a credible source? Both UESP and The Imperial Library say that Marukh was a 1st Era prophet who wrote the Alessian Doctrines.
Wulfheart was a Nordic King, a general, a Storm Voice, but also an avatar of Lorkhan (thus Lorkhan). So simply asserting that Marukh was a first Era prophet doesn't counter this. In addition Sload is knowledgeable, more then most even, so it's atleast worth a short consideration.
Though Whitestreak really should do more then simply referring to Sload. At best reproduce the argument, or at worst quote Sload.
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Anyway, I've asked Sload. His reply:
"While marukh is definitely a first era prophet, Sload of TR believes that he became lorkhan during the dragon break as explained in http://www.imperial-library.info/fsg/azurastararticle1.shtml"
He also linked to this: http://www.tamriel-rebuilt.org/forum/viewtopic.php?t=20460.
Seeing there was nothing there about Marukh becomming an Aedra I pointed out that Selectives came a long while after Marukh was dead, he replied:
"He [Whitestreak] doesn't know the difference"
So I assume that Whitestreak is has mistaken the Marakuthi Selective for Lorkhan.
I'm a bit puzzled here because I expected some argument along the lines of identification by incarnation, but instead Sload seems to assume Whitestreak is mistaken.
This puts everything back to the point where we ask Whitestreak to clarify his argument.