The pneumag is a mechanical gun - in the sense that we judge mechanical guns in paintball. I guess technical it is a pneumatic-mechanical gun and there might be some discussion if it is a "true" mechanical gun, but I would assume the same discussion then applies to autocockers.
The problem I have with some people who insist they are "mechanical" is then they beg for mechanical only tournaments. These same people sometimes bemoan the rise of BPS in the sport and then seek to make the rules such that they have the BPS advantage. Up until this point I really don't worry about how one classifies it.
"Unless someone like you cares a whole awful lot, nothing is going to get better. Its not" - Dr Suess