Laenor got his dragon riding from his mother, Rhaenys.
Honestly I think the show is at least hinting that you might not need to be a Targ at all to ride a dragon. The dragon just needs to like you since we don't know if Ulf is a Targ bastard or not and who knows how far back you got to go to find some Targ blood in Addam.
I forgot Laenor was Rhaenys' son to be honest x(
I mean I guess we'll never really know if Ulf was telling the truth unless the show tells us explicitly. But in this universe I don't think it's a stretch to think there's Targaryen blood throughout Westeros, however diluted. Would be one thing if the Targaryens only procreated through incest but that's clearly not the case.
But given trends I'd be inclined to believe that Ulf has bastard Targaryen blood some way or another. There's only four "surprise" dragonriders across both shows and three of the four are either implied to have or directly have Targaryen or Velaryon parentage:
Alyn: maybe not Targaryen blood but he's Laenor's half brother
Jon Snow: Rhaegar Targaryen's son
Hugh: Targaryen mother
Ulf: claims to be a bastard brother of Viserys and Daemon for bar clout but gets nervous any time he's in position to actually be "found out" or challenged about it. Maybe true.
Feels like with the first three being a direct Targaryen descendant or a relative of a dragonrider helper the eventual riders. So I'd be inclined to belive Ulf was telling the truth to some degree.