As someone in the Ultimate modding community, this personally has me convinced.
The big thing about it is just how differently Mii stuff works in Ultimate vs. Smash 4. In Smash 4, Mii outfits directly scaled with the size of the Mii in question, which was a quick and easy way to make sure they all looked decent. However, this wasn't obviously the best solution, as it meant that a lot of Mii costume parts would clip into each other pretty badly and not look great as a result.
In Ultimate, they changed that. There's now a specific sizing algorithm for each individual Mii piece in order to prevent that issue from occurring again, at least as frequently. That algorithm is hard-coded into the Mii characters themselves, meaning that in order to find it, you'd have to dig through the entirety of each Mii's code to even get started on Mii costume mods. And in the video, the hats are on two different Mii types, meaning they'd have to do that whole process twice.
In addition to that, there's a notorious filesize problem that's severely limited Ultimate modding so far. If a modded file is bigger than the base file it's replacing, the game will crash upon trying to load that file. So if you have a 2 megabyte file and replace it with a 2.1 megabyte file, the game crashes. This is notable because, without the specific tools Sakurai and co. use to compress these files, modders have a very hard time shrinking their high-quality mods down under that limit. That in of itself is why Ultimate modding is so slow. Now, consider the fact that Mii costume files are tiny compared to actual fighter files, and it gives serious credence to these high-quality modeled hats. If they're mods, they're not only better modeled than any Smash 4 Mii costume, they're also several leagues smaller in terms of filesize in order for them to even be able to work in Ultimate.
So yeah, I'm #teamreal on this one.
