Mother 3 was on a handheld. Its stage is from a console game. If Xenoblade doesn't have a stage, then it and Wii Fit will be the only series without stages on the 3DS version, which just seems off to me.
It says ESRB because it's being recorded and shown to the ESRB. And while it is from the English language version of the game, hence Bowser, you can pick languages in Smash regardless of where you get it, and it changes the names accordingly. So it could be, and probably is, a Japanese copy being played in English for recording purposes.
Although, honestly, I don't see why where it's from matters in a leak like this.
FAKE EDIT: Just now realizing that the original Mother has been on a handheld, so yeah lol. Still, would be weird for Xenoblade and Wii Fit to be the only series without stages on the 3DS.