Interesting and I hadn't picked up on it entirely, but saw an article that discussed Stan's cameo in this one. I did get the reference (Mallrats),but not what it meant. Since it references a film where Stan appeared as himself, that would suggest that he was actually himself in this film. The article mentions that this is the only time that has happened as opposed to Stan being a random character.
I loved the Stan Lee tribute opening scene too. People clapped at that and at the cameo.
He has one more: Endgame.