So? Smash has always been a 2-4 player party fighting game first IP and that doesn't stop the single player campaigns (multiple, even) from being some of the better examples in a genre that trees them like an afterthought.
Just because the single player campaigns in other Multilayer Focused shooters have been unmitigated bum doesn't mean Nintendo follows in lock-step.
Same case goes for something like Brutal Legend imo.
Schafer used to talk about how the single player was just a "tutorial" for the multiplayer but I never played the multiplayer more than once because it was just boring. The singleplayer at least had fun gimmicks and a cool narrative that made the playing a lot more inspired.
Splatoon doesn't seem to have much of a "narrative" but the execution of single player looks a lot more inspired than most of its multiplayer shooter peers. Even if one thing is hyped up as the main feature doesnt mean that another significant one can't be a selling point. And if people keep trying to level at it with some kind of "well I bet its just AI bots" argument that I've seen quite a bit here in the last few threads, then that kind of tells me they weren't interested much in the first place to find out.