It's... weird. They said core title or somesuch, but have also elsewhere referred to it as a title for newcomers rather than the hard(er?)core audience. It feels like they are trying to market it as both a spin off and main title, which is just confusing.
Maybe it is best to consider it as something in-between. It has the battling mechanics to some extent - but no clear sight yet on how items work or if abilities exist, and stats may be different (CP is in there after all). It's based on Kanto, yet it has clear GO elements in the redone catching part. It's by Game Freak, but that's hardly reason to claim 'main game' when they've done spinoffs and non-Pokemon titles (Pokemon was not their first bit of VG work on that note).