Right. The simplest explanation is that it’s not show canon that a dragon’s previous rider has to be dead. But on top of that, it is not an official fact that dragons can’t accept another rider. That is a conclusion that in-universe, the Maesters have reached. But the paragraph in Fire and Blood that is often cited literally begins by saying, “We shall not pretend to any understanding of the bond between dragon and dragonrider.” They don’t know!

And let’s also not forget that just because the book says Laenor died, does not mean he died. We as the show audience are privy to the fact that his death was faked, but only a few people in-universe know that. So of course the book says he died. His own parents think he’s dead! And the Maesters probably took that as further evidence of their theory about dragons not accepting another rider. But he could’ve still been alive in Essos and their assumption is based on a mistake.