Yeah, no it’s a self-contained campaign which can be played single-player or with a friend online. Maybe that part of the game is gone? I don’t know. This does not strike me as a game designed specifically for cooperative play though.
I completed it solo last year shortly after launch and it’s a solid and amusing Metroid Prime-esque first-person adventure. There’s some DLC, Hot Garbage, but I’ve not checked that out yet.