So, bear with me for a second and let's say the leak is real. For a lot of us, the big cause of doubt is that 5 more unique newcomers and 2 echoes is a LOT, especially since we've gotten used to each unique newcomer having a custom trailer.
But what if they're all (or mostly( introduced as part of a big blowout trailer with multiple reveals, ala the initial Brawl and Sm4sh trailers? Iirc the Brawl trailer had Wario, Pit, Meta Knight, and Zero Suit Samus. We could have a similar trainer at the beginning that introduces Isaac, Chorus Kids, Mach Rider, and Geno (for example), Ken and Shadow quickly revealed partway through the Direct, and then Banjo-Kazooie get their own trailer at the end.
Just hypothetically, that would at least be a more elegant way to reveal everyone than 5 separate trailers shown back to back.
Or as others have said, some of the characters could be DLC.
Regardless, I really want to believe this one so I'm just going to let myself live in this happy world until something definitively disproves it. It'll certainly make the next few weeks until we get some official info easier to bear.