I think the point is that the bladder is above where the pee comes out so gravity could be pulling it out of the body. Aiming upwards wouldn't change this relationship because the pee is outside the body at that point. By being upside down the bladder is now below where the pee comes out and gravity would be keeping it in. By being able to pee upside down, he demonstrates that gravity is not a necessary component.

I think it can be demonstrated by aiming upwards with some extra pressure though. If gravity were the only thing pulling pee out, i wouldn't be able to shoot over someone's head for instance as the pee wouldn't have enough kinetic energy. Thus gravity is not the only component.