There's no way this is what the actual full game will run at when it releases. Nintendo wouldn't let that slide. I mean, the first Splatoon at least 720p even on the Wii U and we know the Switch is a lot more powerful than that. It's not even a demanding game, at least I don't think so.
Mario Kart 8 Deluxe runs at 1080p, and I would say that looks easily more graphically demanding than Splatoon (Even though Mario Kart 8 Deluxe is a Wii U port, Splatoon just doesn't seem as graphically demanding, it seems more simple than it). It would never make any sense for this game to run at that resolution other than that it only runs like that for the testfire and not the actual full release.
Probably just like that for the testfire, because it doesn't make any sense otherwise for the game to be running at that.
If the full game releases and it's running like that, then what the fuck Nintendo.