didn't the Teraleak mention that Greece would be used as a base for the Gen 10 region?
Absolutely nothing of the sort. Greece, especially the Cyclades specifically, is pure conjecture spewed by Khu and co. to get them more attention. The only actual info we have on the region is that it
might involve some kind of travelling, thanks to a single (roughly-translated) conversation between higher-ups at GF and anime producers being concerned that Horizons' themes might overlap with Gen 10's.
From what I can recall, this conversation was either in late 2023 or early 2024. It wasn't extremely recent. Gamefreak have had a lot of time to change things since then, and given that Gen 10 would have likely started development around 2021 or 2022 I can imagine it'll look very different now. There's a high chance any kind of inter-region or inter-island travel was cut, either for cost-saving measures or time, since we know they've done this before with Alola and Ohmori is a hack.
The Teraleak confirmed that the code-name for the region was Gaia, and that the Japanese names
likely started with a K and an N. From what I understand, that's the only outright confirmed information out there and it tells us absolutely nothing.
Given that Scarlet and Violet's codename was Titan, I can see there being some kind of focus on the world for Gen 10, but it might be just one part of the game or simply describing an overarching theme. Hell, it could just be an artifact of them attempting an "open world" again and deciding on the code name so early in development that they didn't have much else to go off of.
Or it could just be meaningless nonsense. Nothing about Ultra Sun or Moon had anything to do with narwhals, after all.