>>37070334>It's officially acknowledged and labelled as a Mainline Title by GameFreakWhich might mean something, if they hadn't started calling their OTHER series Core (even going as far as to explicitly separate LGEP and Pokémon 2019's Core games (and further sticking a pin in that ballon, by equating Pokémon 2019 as being similar to SM/XY/all their previous work.
When even Game Freak are calling it something that's not the same as their ACTUAL mainline games, it can only be a spin-off (especially, as you rightly pointed out, it uses mechanics inspired by an actual Spin-off game already...)