I don't think this is right. What causes the problems in Smash is the fact that it uses bad and outdated delay-based netcode instead of modern rollback netcode. You can go hop on any number of p2p fighting games with much better netcode than Smash and feel the difference immediately
It's not feasible to use any other form of netcode for Smash but delay-based lockstep with the features and gameplay that Smash supports, which go way beyond other fighting games. I know gamers think they know better than Bandai Namco's engineers, but they don't, and it was even tried during the development of Ultimate, and it didn't work.
It's much easier to do it in other fighting games that consist entirely of two characters on a flat plane doing nothing but shuffling back and forth and jumping. In those games, you also don't get to see the negative outcomes of those other kinds of netcode as much either, since they sell 2 million copies if they're lucky, and are played mostly by an older demographic with good wired Internet connections. But if you play a match with a bad connection, then sure, the game doesn't slow down, but the characters start teleporting around instead, and already completed actions get undone and you find out you're already dead from a hit that happened half a second ago.
Also note that Smash is a great example of people complaining about "Nintendo's netcode" being a red flag, because Nintendo doesn't develop Smash.
Nintendo standard libraries for managing those peer to peer connections are, indeed, of very old vintage
This is a perfect example of casual misinformation. You're thinking of NEX, which was Nintendo's old system for managing
lobbies and matchmaking, which was licensed and developed out of Quazal Rendez-Vous, a library that has been around since 2003 or 2004. At some point somebody found a string mentioning Windows 98 or something in an NEX binary, and that became shorthand for people to laugh about how old and bad it was. I think you know as well as I do, that's simply an unserious avenue of criticism for a piece of software. To illustrate that nicely, Rendez-Vous's initial release was a few years
after the 2000 release of Havok Physics, a library that Nintendo similarly adopted and built on and still uses today, perhaps most notably as the foundation for all the crazy things you can do in Tears of the Kingdom.
But that has nothing to do with netcode, since NEX doesn't handle peer-to-peer gameplay. Nintendo has an in-house library that does that, called Pia, which debuted somewhere between the launch of the 3DS in February 2011 and MK8's release in May 2014. People don't have the "Windows 98" meme to fall back on for this one, so the only criticisms you see of it are "the netcode is bad" as a catch-all devoid of knowledge or merit. Pia works on exactly the same principles that the entire rest of the industry uses for netcode. There's nothing remarkable about it, and nothing lacking. It does what it's supposed to do, and when people have good internet connections, it works great. When they have bad Internet connections, it works badly.
On top of that, Nintendo isn't even using NEX anymore. They created an in-house replacement called NPLN which has picked up all new development and features and is now the recommended library for lobbies and matchmaking. So to the extent that the "vintage" NEX was hampering online play in Nintendo games -- which would have been in the realm of leaderboards, lobby features, and matchmaking methods -- that issue has been addressed.