Splatoon 1 had the same Global Testfire period on the Wii U, and it was to test servers and whatnot. Pretty cool they're doing it on the Switch again.
As for game demos, I wouldn't count on there being too many. It was proven long ago that game demos actually decreased game sales, and because of that, it's very rare to see a game demo nowadays. Online-focused games like Splatoon almost always have a beta test of some kind, however, and that's as close to a demo as you'll usually get nowadays.