It’s not an intelligence thing, it’s a function of psychology. He’s in denial about how much trouble he’s in. Who wouldn’t be? It’s hard to admit to yourself you’re facing years of jail time.

Running his mouth is his way of living in a reality where he still has a measure of control over things.

This is more on the spectrum of delusion and delusion has nothing to do with intelligence. Very intelligent people can be highly delusional.