It does look like the art style from Returns, just with DK using his Smash design instead. Too bad Eggman completely screws up any chance of it being remotely legitimate. Yes, I'm not going to try to shut down leak talk in here anymore, since we're literally days away from release.
Actualy, they have a curious point though, in regards to the clips we saw, I think we may have seen our first looks at the character videos without even knowing it. I mean Sakurai said something along the lines that he was going to do character videos, with each character interacting with the other characters. now technicly all those clips we saw on the reveal work on their own, however if you take them as a link to the older intros, or the games themselves, well a couple of them seem rather obvious.
Samus: The clip is extremely similar to the one in Melee in which she is attacked by Ridley(and hasn't he been in consideration a loooooooooonnnnnnngggg time?)
Pit: a callback to Brawl where he is summoned by Paluneta, who has been confirmed
Fox: Again a callback to Melee, where he was attacked by Wolf, who was considered for Melee, but added in Brawl, now it could once again be him facing down with Wolf, but Wolf hasn't been revealed yet so Sakurai cut before that part
DK: Now this is curious, yes its his standard running out setup, but he's not just whipping up leaves, if you look closely he's whipping up a trail of BANNANAS that he's following, now while that happened in DKCR, there is no way the head Tiki is going to be playable, and it is more likely a reference to DKC where a certain Kremling King took off with his Bananas leaving them scattered all over the place
Now its tenuous at best, but it could be a clue, and it could be part of something bigger, all of these clips could easily fit into a bigger 2-3 minute character video, possibly involving other characters who are playable
edit: Oh, one other observation, we probably won't see these videos till the wii-u version, they would take up too much space for the 3DS version, and wouldn't look as good, and would be a nice treat so you get something for unlocking characters in that version