These phenomena have been documented, in-depth, for many, many decades. Credible sources note that certain materials and devices (I don't want to use the term 'craft') have been in government possession for going on 90 years, since the end of WW2. The notion that some country had achieved technical supremacy such that we still find their technology unidentifiable for 90 years isn't tenable.
Everyone involved in what the public knows about this are largely credible people who have undergone a classification briefing of what information they have, carefully vetting what they're able to share. They seem to feel that this topic is relevant to Americans and their civil liberties, they simply don't want to go to jail. I would tend to agree with these people.
Now, the government can actually just say whatever Dave Grusch knows about these entities can be declassified. Just say, "Dave is not bound by classification for any claims relating to contact with entities, deals with entities, specific information about entities, etc." This would instantly discredit his claims, because he'd be free to make outlandish and absurd claims without being able to hide behind the veil of "it's classified, we have to discuss this in a SCIF".
If they don't exist, what's the problem? This is a man we know was at the highest levels of the actual stuff he's talking about. He's a credible source. If he's making outlandish claims, just lift the veil of secrecy.
Of course they don't do that because it is classified.