After getting to the end of the game, I'll believe you. But if you mean starting a new game and from the start of the game you did that in 20 hours then yeah, not possible without hacking.
Not true, there's a very useful XP gathering area in the Pride Lands, on top of the mountain where you fight Scar. Equip the Gullwing Keyblade, and equip "XP Boost" and get your HP in the red. Equip Donald and Goofy with weak weapons and turn off their abilities, because you don't want them to kill things.
On top of that mountain is a crapton of those yellow flying things, and they give good amounts of XP with those two abilities. I went from level 60 to level 99 in about two hours with this trick. So I believe him when he says that.