The game is still marketing itself as a post apocalyptic open-world MMO. Took this screenshot a few seconds ago - https://store.steampowered.com/app/1372880/The_Day_Before/

Also, once the trailer narrator starts, she also says "post apocalyptic mmo open world survival"

I mean, maybe technically it is those things? Only the world is super small and you can only access it during a match, which happens with, dunno, at least 50 other players, which is "massive"? It's more players than what most WoW raids or pvp matches have.