Who's Fatmanonice again? I wasn't active on here until early 2020 (I had nothing better to do during lockdown lol), but I watched a lot of speculation videos and looked into some stuff with friends (or, well, friend), and the name rings a bell, and not in a good way.
The thing is, if this was just presented as speculation without their name attached, I'd honestly say it's worth considering. Nintendo clearly doesn't have much planned between Mario RPG and the next console, other than the Peach game (which feels vaguely unimportant with how casually they announced it), ports, and switch online stuff. Nintendo would want to stay in the media and public consciousness in the lead-up to a new console, and new smash reveals would be a great way of doing that, with next to no effort from their major teams.
I don't think it's happening, but, for the first time since Sora, it feels to me like there's a (very) small chance (like, 0.01% or whatever) of there being new smash content. As soon as the Mario Kart DLC was announced I realised that Nintendo must have at least considered making more smash DLC. It's basically free money. And now that it's pretty much over, they'll almost certainly want something similar to fill the gap until the next console.
I'm not sure if there's any way to check, but it would be interesting to see what the smash DLC team at Namco are working on right now. I'd assume Tekken 8, but that was in development before the smash DLC ended, so I'm not sure.
Anyway, if anyone gets their hopes up over this they're just setting themselves up for disappointment. I want it to be true as much as anyone else, but I think expecting something like this is a huge mistake.