This one was almost implied. Imagine you've gotten a bunch of figurines of your favorite characters from Nintendo games. Joy! And what else to do, but make them fight with each other! The "brawl remixes" are songs in the boys imagination, which obviously can't be the exact same as in the games. The boy's favorite game would be Earthbound, which is where "smash" would come from. Subspace Emissary is the boy's own fantastical story, with his own villains and enemies. Master Hand represents creation, Crazy Hand destruction.
It almost makes too much sense.