Not sure if I would go all out and call it a "mode". There are no accommodations made to make it balanced or fun in multiplayer; it seems likely there for testing purposes rather something they intended to offer as a secret (and Nintendo Power would obviously have "leaked" the cheat if it was something Nintendo intended kids to find.) I'm going to assume it doesn't play well, since every other character has a variety of moves and Mac can just super-uppercut if he manages his meter. (I've seen Punch-Out-type games or hacks do multiplayer elsewhere, and it tends to underline the fact that Punch-Out is sort of rhythm-game mechanics in a boxing shell, and what makes it fun is what wouldn't work if it treated gameplay as "real" boxing in like a multiplayer scenario. Punch-Out Wii messed around with multiplayer, to some degree of success.) But it's an amazing find nevertheless!
The whole idea of this makes me wish Nintendo would come out with super punch out mega edition. 50 fighters, some new, some from nes. Custmizable and upgradeable fighters. And a whole true mutiplayer online mode.