Again, around 5-6 hours is not very short single player campaign for online focused game, it would be for single player focused game. You can scholl player in 10-15 min tutorial, you don't need 5-6 hours full singleplayer campaign for that. They can sell Octolings without another single player campaign. Reality is that they just want to make Splatoon 2 more appealing for single player players also despite game is online multiplayer focused.
MP4 can easily have solid online multiplayer mod and keep focused on singleplayer, similar like Splatoon is online multiplayer focused game but also has single player content. Also, becuase paid online Nintendo could force online multiplayer for MP4.
When the likes of COD hav 7-8 hours and its considered short then 5 hours falls into the same boat no two ways about, you can school players in many ways it still doesn't debunk that the single player is mainly for players to get acquainted with the unique mechanics.
Splatoon's single player is nothing to write home about so it having a single player doesn't really help your point especially as the vast majority of player don't bother with it right now your argument hinges on a what if scenario that Prime 4 can have both on equal level historically games in recent times have not managed that with the higher demands of development it's not as easily achieved as you say for a start Prime's design doesn't translate to MP well.