I had a reply from the guy I know.
His credentials:
https://www.open.ac.uk/people/telc3
My Question:
Hi Tom, hope you're doing well mate. Quick question, David Grusch, credible or not?
His reply:
Honestly, I don't know. Not the first to make this claim, though perhaps the one with the most credibility to lose. All publicly available info suggests there aren't any extraterrestrial intelligences in any "reasonable" travelling distance from us (assuming no faster than light tech) and if there were I find it really hard to believe radio astronomers have sat on that. My gut tells me this is a misdirect- the USAF in the 50s felt that letting the UFO mania become a thing was a sensible PR strategy (both to distract from testing their aircraft and in case any of these UFOs were soviet they'd prefer people think they were aliens rather than soviets) so a similar thing could be happening, and honestly I find that more likely than we're being visited by aliens and the government has kept that a secret but I can't rule that out.