Ok I watched the video... And it's french detective time!
I participate a lot on jeuxvideo.com and I believe that it's just bull****. The first claim that presumably leaked Richter and K.Rool was something just forged, it's rather easy to do by modifying the HTML code... It's only you that see it, of course, but that's an easy screenshot to do then. But there was a flaw in a second screenshot made as a proof: a space has been added before a Colon, right after "Il y aura en persos jouables".
See for yourself:
Secondly, I know that the 18-25 boards are really really active on "JVC" but why the hell that leaker wouldn't stop by on the SSBU forums? Even the "console war" board would have been a more logical place to put it. (yes, we do have a console war board... And that's troll-filled to the brim).
Finally, the content. It's truly fishy. Not because of the characters themselves, but mostly because... Strangely that's characters we talk about a lot on the SSBU boards. Obviously Isaac is just a fan favorite everywhere, and we do have devoted fans as well. Skull Kid is basically a meme on our board at that point, considering how pointeless debates about him are... But Medusa? We happen to talk quite a bit about her. I admit it's a bit because of me, but not only...
We tend to mention Ayumi when talking about retro characters as well, but not that much.
What tick me off is mostly Reimu and the Mother content... See, we have one guy here on the boards who loves three things: Skull Kid, Mother and Touhou. And let's say he is... Really pushy about it. No to say annoying. If I said that Skull Kid is a joke at that point, he is part of this (although people against SK do add to that joke).
But a french leaker who claim that there's Giygas as a boss (something he'd like), Ninten as an echo, Skull Kid as a newcomer and last but not least Reimu? Two possibilities: Either it's this dude (he claim it's not him) or it's just someone else trolling, basing his list on our board. I'd dare to say it's the second option, as the other guy is also the kind to believe that there will be something like 15 new characters...
Also, we are basically mocking it on the SSBU forum on JVC... The only thread we have is basically "Who did it?", so you can imagine how much we take that seriously...