This year's tournament looks quite awesome.
Why is Harry Potter a super?
The same reason Doom is I imagine. He may actually be human, but he possesses extra powers which make him far superior to any standard human.
Bingo.
If you look at the human bracket everyone there is naturally talented or has equipped themselves with technology that makes them a threat. They're just normal people, but highly skilled. Harry Potter was born with an ability that few others possess. He's basically like a mutant in the Marvel universe: there are many of them, but they are radically different from the overwhelming majority of the world's population.