Nonman can't be mistaken, and Bekker isn't that kind of writer to pull out "piano in bushes" to somehow eliminate the fact he stated before.
As far as I can tell, wrong on both accounts.
In fact, Nonman are terribly susceptible to mistakes. They only have parts of memories and the Erratic only remember tragedy anyway. He could have just thought he looked like an Anasurimbor but that doesn't make it so.
Also, we know that Bakker specifically has wrong/misleading information everywhere in the books. At this point, those of us around here like to believe that much of what is written can, and is, plausibly false. (Coming up with reasons for out crazy ideas has become quite the pastime)
Character accounts are flawed, since no one character has all the information. Achamian is wrong an awful lot.
At one point in WLW, just before the Coffers, Cleric thinks that Achamian actually
is Seswatha. That right there is an example that points to the potential inaccuracy of both your statements.
One last thing, the Appendix has some incorrect info in there, and that isn't even part of narrative. In that case, Bakker is doing the misleading.
None of that really answers your original question though. How about this: adoption and/or infidelity. We don't know a whole lot about the Ganrelka line (or at least I don't). It is possible that somewhere along the way his line was not "truely" Anasurimbor.
However, I doubt it. I think Kellhus being Anasurimbor is extremely likely. Though it also likely that he is not the one the prophesy foretold of. He might not be the Harbinger.