Then there was also something that came to my mind when speculating the worlds. Maybe all the returning worlds Olympus, 100 Acre Wood and etc doesn't fill the list order. Since they've already been in previous instalments? This I mean for the discussed sections of early, middle and latter worlds. What if all the sections only involve new-never introduced worlds?
I think they're just talking about 9-10 worlds, disregarding if they're new or not. It's just up in the air if they mean 10 total worlds or 10 Disney worlds plus the original worlds, and if worlds like Mysterious Tower count as it's own world (which I would assume not, because in KH2 it was a part of Twilight Town and was considered a subsection of that world). I'm not even sure if 100 Acre Woods would count towards this total or not. It's usually just minigames that you can skip if you wanted to.
They still need time to develop the worlds after all, and Olympus has plenty of new areas such as Mount Olympus and Thebes. Why wouldn't they develop it alongside all the other worlds, just because we saw it in another game when KH3 is using a different engine?