Fallon: "So youre Vin Diesel. Now you're Mr. Cool, but you used to be into D&D. Exactly where did that switch flip?"
The audience and Fallon laughs each time he mentions something nerdy as if it's unbelievable and/or funny.
Fallon: "I can't believe you became who you are."
Fallon is clearly asking the questions to get a laugh out of people based on the perception that "D&D and WoW are for losers" and Vin Diesel isn't.
So, you can all go suck the metaphorical moose wang.