> Loeb could not have been any more mainstream or credentialed, yet here he was, saying that maybe an alien spaceship had arrived.

Speaking as a former physicist (but not astrophysicist), at least to us practicing physicists, someone being well credentialed hardly lends any credence to their more out there theories. I know multiple renowned professors in high energy and/or cosmology, including at least one in my department (Princeton Physics), who have been pursuing more wacky theories late in their careers that others in respective fields ignore for the most part. In fact, being well credentialed probably makes it easier to gravitate toward the fringe, since there’s no more normal career objective to pursue. No comment on this specific case.