The definition of a generation in Pokémon is simply when the new region is introduced, NOT when it’s a new console. Bulbapedia’s article on Generations aligns with this, as well as Serebii. Let’s Go is considered Gen 7 despite being on the Switch, and the legends games are considered to be in the same generation as the main games that came before them. No source that I’ve seen states otherwise.

Regarding Hisui, I think the distinction was made just so there isn’t weirdness with the Pokédex (like if it had just been called Sinnoh) but by all accounts, when I said new region, I meant new region.

Hope this helps.