why would they video tape if they didn't believe it gave them an advantage
Fixed.
This dude lives in a world where the Patriots were just out there videotaping teams for fun and didn't use the information to attempt to gain any sort of advantage over teams. Add in the fact that he believes other teams did it as well. So apparently multiple teams were committing rule violations with no apparent benefit.
I don't understand how you can simultaneously make the argument that other teams were doing it and also that it didn't give an advantage.