Questioning someones else skill in BJJ is something I'm sure he takes personally.
Plus Vinny is better then him in BJJ so him saying that probably hurts his pride a bit. Plus I'm sure he doesn't want to be disrespected.
Vinny's BJJ is great, but better than Nog's? Maybe in strictly BJJ competition, but I'd wager Nog's the better grappler in MMA.
How did Nog do in BJJ competitions anyway? Did he even bother with them? He's grappled against guys that have won ADCC and done just fine.