While yes the other things that people pointed out that makes this video fake/edited are true, if nobody noticed them and believed it to be real the underlying thing that would seal the deal as fake would be the part where it "freezes". Games don't freeze like that, much less Brawl. Brawl, when using codes and doing something the game can't do naturally and it freezes, you hear a buzzing sound. No sounds are repeated constantly, the song doesn't stop at where it is, when the match starts and it happens to freeze on "GO!" the announcing sound doesn't repeat with "G-G-GO-G-GO-G-GO!" It just either has a loud buzzing sound OR no sound at all.
If he really wanted to make this believable, he'd get the buzzing the sound instead. That's what would make me believe it was fake if everything else was actually pretty "real". But, they aren't, and so this is just another piece of evidence that it's fake.