That’s true in a trivial sense: there’s no law of nature that enforces verisimilitude in any work of fiction. However, most authors aim for verisimilitude, and the good ones achieve it. I’m not talking about the top speeds of balrogs because I think there’s some objective answer, but rather because I think that Tolkein does achieve verisimilitude (at least in some regards) and therefore there is a foundation for discussing the traits of his fictional beings. He easily could have given balrogs rocket skates, but he didn’t.