I’m SO glad that Baelon didn’t cheated on Alyssa and stayed loyal to her but narratively, Ulf being non Valyrian just doesn’t make sense. Because why would he lie ? Why would he risk his life to make the attempt ? If everyone believes you have to be Valyrian to be a dragonrider, how would he know he could ? Does he just feel the ability inside himself ? Absolutely nothing makes sense in this garbage fanfic.
I did not watch S2 E3 and I shan't, so I do not know what exactly Ulf says, but I'm inclined to think that he says whatever he says so that either he can just feel bigger & get some chance of respect/care about his life from people around him than his measly life (sometimes people be stupid, and his book self was certainly that; the drinking habit would worsen his tongue and loose faculties) OR/AND he is aware of the idea of dragons only being able to bond with those descended from Valyrians/Targs.
And it is a society-held belief that Targs exclusively can ride dragons through their blood. Has been for years before Rhaenrya was even born. If Ulf knows he had a Targ parent, he can just, yes, more material to believe he could ride a dragon.