My point was he can still keep them at bay because there isn't very many safe opportunities to attack MK in the air. Heck all he need to do is neutral B to shut down most air games.It does matter. Mobility is huuuuge in Brawl(Sonic is a mid tier despite being one of the worst in theory. Purely because of how mobility grants him amazing options)
A normal scenario in the air is weaving in and out while occasionally attacking. If your mobility is great, you can back off after whiffing a move without getting punished. Those with lesser-than-great mobility cannot.
Having a big, fast, disjointed, transcended hitbox is obviously going to help a lot, but I'm just stating that it wouldn't be an obvious winner in such a scenario(Especially if you start thinking about projectiles. Transcended priority cannot stop them because it completely ignores them)
Regardless Metaknight wont take them on in the air if he doesn't have to (beyond the typical short hop Dair) and when he does its off the stage where he clearly wins