Was just about to post this. The moment a leak brings up the trailer you already know it's over.
The fact that it regurgitates the "le bowser jr alts" meme without even mentioning the other characters' names is the final nail in the coffin.
It didn't actually mention what kind of alts they are.
It's more of a Hero situation since it's based upon Spring Man. Bowser Jr. requires a Mech and the Koopalings actually have similar, but not the same, bodyshapes. They're closer to Villager and Isabelle. Without the Mech, they could not work as alts to begin with.
Hero had the same bodyshape for only 4 characters(they could've had other female Alts, yes, but not for everyone, heh) entirely. In order for this idea to work, you need 7 more characters with the same bodyshape as Spring Man. Without Mechanica's suit, the only way a Bowser Jr. situation can work, it's not really possible to do all 8 at once. It's a blatant fake.
Also, I wouldn't say they can't know the trailer. It's barely a month beforehand. Them being leaked the trailer is not remotely impossible. The trailer was clearly recorded. It's still fake, but not because of the trailer, or the date(lol, that's a super believable date, honestly. It's also hyper easy to predict and means nothing in return. Other than them getting the year count wrong, it's a perfect day to release), but because you can't possibly have a Hero situation here with 8 different characters.