Grusch is documented as being in a position where he could have access to the sort of classified information he claims to, and his background suggests he could interpret said information reasonably as it pertains to this. He has been vetted by congress. Although it is certainly not conclusive, what we do know about him corroborates with what we’ve heard so far.

But doesn’t all his knowledge come from second-hand accounts? If so, why should his opinion matter any more than anyone else who’s read someone’s account of being abducted and probed by aliens? Working for the government and having access to classified material doesn’t make his claim any more legitimate if all he’s doing is reading stories written by others. It’s the appeal to authority fallacy in a nutshell.