I mean maybe, but like Nintendo randomly took down a Kos-MOs mod for no reason.
Aand costumes are pretty easy to write off considering Nintendo knows anyone could mod them in, so if it is real, why bother taking it down and confirming two big characters?
At the end of the day I don’t think humans actually strike these types of videos but an algorithm that doesn’t always work right.
That's not realistic. They don't really care if it confirms something or not. Otherwise they'd constantly ignore any online leak. They take them down because it breaks the NDA and should not be up. Mods are more iffy legally nor have any stigma attached to them, so it doesn't matter if they take them down or not in general.
Keep in mind NDA's actually matter to them because if something breaks it, they found a non-trustworthy employee and they want to get them fired. There's even a rumor they fired the Ken leaker too. Which was just an image, not actual video gameplay. They've been massively cracking down on leaks for a vast while now. There's no way they'd ignore a costume leak like that. Never mind you also have other companies who'd care. Square-Enix and Betheseda ignoring it too? No. That's not happening. It was ignored hard because it was a mod. This is a case where there was no chance it could be real. They confirmed already that leaks in general are a pretty big deal if they're video ones. Also keep in mind people rarely throw video leaks on Youtube, where Nintendo sees it. That's the only reason they don't often get taken down. Besides that, all those beta leaks seem to be a different situation. It's not for an upcoming game, but old unreleased content. They went after the website, but ignored Youtube.
Beyond that, video leaks of upcoming content can also affect sales. Since people can decide far too early whether they should buy the content or not. For a company, they want it unveiled in their own way, to maximize all possible sales. Especially since presenting them is done in its own fashion. Whether it could be because of exciting music to hype people, a very good announcer, or any other thing that makes the presentation very nice. Context matters for how things are shown off. Another is that third party companies have made agreements in how to present their content too. If somebody breaks an NDA, it makes the company look less trustworthy too, since they aren't holding up to the agreement that was made with negotiations.
Basically put, leaks are bad most definitely. And Nintendo quite clearly does everything in their power to kill them(again, for upcoming content in particular). They have lots of good reasons.