I guess I don't understand what the fundamental disagreement is here. Most astronomers believe the probability that Oumuamua was an alien visitor is very small. Loeb's argument is that the probability may be a bit larger. But we'll (very likely) never actually know which it was, so the disagreement is about the unknown likelihood of a single observed event. It just seems obvious that we'll have to settle for "probably a funnily-shaped rock, but there's a small chance it was something more interesting" and live with that.