What doesn't fit the IP? The idea of a multiplayer shooter in an IP which has had both multiplayer and shooting? The idea of zombies and crazy scifi in an IP which has had both zombies and crazy scifi? The idea of a different genre/demographic appeal in an IP which has had spinoffs in the form of card strategy games, a mobile shooting gallery, and a mobile social card game?
Wormholes were in mgsv. Alternated universes were in mgs2 substance. Zombies (or at least something similar were in 4 and V. This isn't too crazy for metal gear standards.
The thing about MGS is, people like to go off and say "oh why can't x happen, it's metal gear nothing's too crazy". They write off the plot elements and turn off the part of their brain that makes decisions about what could happen vs what couldn't. Yes, there's giant robots, psychics, telepaths, parasites, nanomachines, etc. But with MGS,
any idea is too outlandish and doesn't fit the series, until it actually happens in a game. Of course a wormhole opening up over MB randomly sucking soldiers into a zombie-world is outlandish. Of course it doesn't fit MGS lore. Just because MG games have included wormholes, "zombie" soldiers, "super-powered" enemies...doesn't mean those things automatically fit each and every way they could be implemented. The IP may have "zombies and crazy scifi", but that's a really broad statement. You have to take things case by case, and in this case, it doesn't really fit.
Now, it's Konami's IP, they can do what they want, they can make this pile of garbage how they want to, and it's supposedly a spin-off, so it's not even necessarily canon. I get all of that, and I'm not saying the game can't exist. But don't just blanket accept every idea under the sun as working in MGS just because the games have "crazy" elements. It's really tiring to see posts by people that think because MGS has tits, robots, and scifi that its world can't be selective about what is allowed (or serious with the story). For some people, the silly aspects become the franchise, and can't be separated from what the games are. Maybe it's just me, but MGS seems to get this a lot more than most games. Wormholes are gameplay gimmicks, not part of the story. "Zombies" have never been real zombies (unclear what they are in this game, but probably not real zombies either), but because some games have had zombie-like enemies, that doesn't mean zombies automatically fit in MGS IP. I understand these games aren't for everyone, but it just doesn't work that way.