What exactly tells you that Ultra Sun and Ultra Moon are the real sequels to Sun and Moon? I couldn't find any evidence of that. At least, not with the official press release:
As if you couldn't tell from the "Ultra" prefix, the press release implies USUM is more akin to an expansion pack of SM. The way they formulated tells that the original story of SM will be expanded, as opposed to a completely new one based on the same settings. And we all know that they're adding more features, too.
With that in mind, I find it odd that they're bringing an enhanced version of Sun and Moon on the same platform as the original games are, yet they didn't took the opportunity to port them to Nintendo Switch in some fashion.
How comes would porting USUM for the Switch hardware takes just as much time as developing a brand-new mainline Pok?mon game? All they need to do is giving each model a higher polygon count and a higher texture resolution and ensuring the gameplay engine fits within the constraints of the Switch hardware. With everything else already done at that point, it should easily release before a brand-new mainline Pok?mon game.