Other people's points for it being fake I'll try to rationalize:
"The DLC team is smaller than the base-game team, and the dude had 2 other people playing with him."
While a smaller team would probably be harder to get a leak out of, it'd still be a big team if they're able to work on multiple fighters and stuff at once. While the Ken leak was with a larger team since it was base-game, the person who leaked that one also had 3 other people playing with them, if testers don't have to control all four characters at once by themselves.
"Why hasn't Nintendo DMCA'd the videos yet?"
Not all ESRB videos were taken down (the leaker's ESRB videos themselves definitely were, but it seems at least a couple of youtube channels' videos are still up from before Smash 4 released, despite included ESRB Leak images and gameplay videos), and they didn't care about doing anything to keep the Ken leak from being noticed. This leak also came from 4chan, and they can't do anything to stop people from constantly sharing it because of that. It's possible that, if their system for taking down stuffs is automated, that's why nothing has happened from them, because they didn't prepare it for this. They did take down music that one time pre-release, but still; perhaps they also can't take legal action like this with the DMCAs and whatnot until they know who did it, which likely happened to the ESRB guy.
"People can mod it!"
The trolls whom claimed to make it and showed off their models couldn't match the quality and detail of the models in the leak, and they didn't show them in Smash. Even that one guy's Cacodemon hat model was missing some details and the texture seemed very simplistic compared to the leaked Cacodemon hat's. No modder has been able to successfully make a hat yet, too, only helmets/masks which completely replace the mii's head. I guess the Cacodemon hat is like a helmet, but since it still shows the mii's head (complete with facial expression) instead of unloading it as another person said helmets did, the game likely counts it as a hat.
"The Mallow hat being jiggly was disproven."
You got proof? How did you and your discord or whatever disprove it? From what I can see, the pirate hat correctly follows the head like every solid hat, while the Mallow hat deforms a bit to the side of the head for a moment as if it has a physics bone. There was another clip where you couldn't really/could just barely see anything, though, so understandable.
The Tails hat may have a physics bone, but it's for his hair. Mallow hat is a bit different.
"The French did it!"
Some trolls in their own discord with no proof, being rude? Doubt. One even claimed to find an old Mallow model for their mod, but no one has found the same model. Not even the Cacodemon's.
"Doomguy is too violent!"
-Bayonetta is a witch, tortures her enemies, is sexualized, has a lot more violence than just the torture...and has demons too.
-Snake has violent stuff happen in his games. If the mere presence of something is enough to not be allowed in by rating boards because of stuff in their game or things associated with them, one of the Metal Gear spirits wouldn't be in the game because of what happened to her and what she references.
-Joker has a gruesome method of summoning his persona, maybe some other stuff too.
-Nakoruru is a mii costume and her game has a lot of blood and whatnot.
-Doomguy himself just focuses on guns in the original Doom (though there is a chainsaw), and he has a soft side based on a few things in newer games. Blood isn't integral to his character, he just wants to kill demons.
That kind of footage is illegal to redistribute, though. Nintendo weakening their legal toolbox for PR is a gamble I don't see them taking.
They can't take legal action if they don't know the people who leaked it.