To me it seems the exact opposite than stressed. It seems completely relaxed, him just being honest about nothing and everything at the same time. Taking his time to think before answering, sipping on a drink and enjoying it too.

I agree he doesn't look much like a CEO, but does that need to be something negative? I think I know Musk better as a person after watching this, in contrary to the typical Silicon Valley "CEO" corporate interviews.

EDIT: Should be noted too that this is pretty common for Joe Rogan interviews. Relaxed atmosphere and 2-3 hours of thinking out loud without much structure or agenda.