I will agree that sometimes he seems like the Worst of Us - amateur psychoanalyzing him, I'd say it's a kind of overcompensation maybe from being the jock in school and the nerds gatekept him out of the hobby?
On his comments about Vin Diesel, not that it matters but I would believe he played before (isn't his witch hunter movie based on his 30th level character he talked about on Conan O'Brien I think it was?) but Joe isn't the only person I've heard say they don't know anyone gaming with Vin Diesel. You don't see anywhere talking about 'Hey! Vin Diesel came down to the gaming shop today for dice and just hung out ten minutes! It was awesome!'. As a multimillionaire, I'm not thinking he's sitting down with us proles. And as far as how connected the gaming community among celebs/entertainers/etc, my only inference is based on a video I saw of Matt Mercer talking about being approached to bring Critical Role to streaming...
The link is from a video Liam O'Brien took when they were approached about the game being brought to stream. The story Matt tells is about people saying they want to play with Ashley Johnson - "oh yeah, she's in my game too." I know it's anecdotal but again... I've never seen anyone lay claim to gaming with Vin Diesel or pictures from his home campaign on the Socials. Video:
Again, all conjecture.
TMDR - I will agree that Joe definitely puts off the 'not-best traits of gamerhood'.
Who cares.
I don't care if Vin Diesel frequents local LA game shops. I don't care who Vin Diesel plays with now or who he played with in the past, I don't care if Vin Diesel plays with other folks in the movie business in LA. I don't care if Vin Diesel plays D&D currently, or if it's something in his past.
Vin Diesel claims he's a big D&D fan. That's all I need. I don't need proof, I don't need verification. There's no reason for Vin Diesel to be dishonest about his passion for D&D, and even if he is being dishonest . . . I don't care. There may be no "evidence" outside of Vin Diesel's word, but there's also no evidence against him either, not that it matters.
Just because you are a D&D fan, and a celebrity, doesn't mean you have any desire to put your gaming out there on the "socials", or need to.
Asking for some kind of proof, some kind of geek cred . . . it's gatekeeping, it's ugly, and it's something that brings Manganiello down in my view for doing it vs. Vin Diesel.