I feel like it probably will be set entirely in Lumiose — while it’s certainly possible that this is just a case of the American Twitter account goofing up (like when they mentioned “Gym Masters” in the lead-up to SwSh), the thing with Legends: Arceus was, the world was divided into six “zones”: Jubilife, the Fieldlands, the Mirelands, the Coastlands, the Highlands, and the Icelands. Jubilife was a central hub, but it wasn’t especially large compared to the wilderness zones. And the only other “towns” to speak of were the Diamond and Pearl Settlements, which were both little more than small campsites.
Maybe GF will somehow create a game that exists at a Hyrulean scale that will absolutely blow me away, but Kalos is a pretty big region, with 16 towns and lots of major landmarks, so I struggle to see how they would make that work without massively downsizing the towns other than Lumiose.
If it takes place in the past, you could probably remove some of them, saying that they hadn’t been established yet, but logically you can’t go very far back in the past, because the concept here is urban redevelopment to create the Lumiose we know.
(I’m not particularly convinced by the theories that it takes place in the future, but if it does, then that only exacerbates the issue — now they would need to render all of the cities and locations we saw in X & Y at a large scale that makes sense in comparison to the size of Lumiose.)
In its current form, Lumiose is actually kinda quite well-primed to be interpreted into five or six major “zones” that could comprise the game’s world. It’s literally already divided into five sections with a central hub where Prism Tower is. I could definitely see the game being set entirely within Lumiose, giving it a strong core concept, with them just greatly enlarging the different sections of the city in order to make it feel big, lived-in, and full of more granular details.
