I appreciate your thought-out theories and observations (you made a Wiki, that takes chutzpah; and like you say "hard to know," and it IS fun to speculate, of course) ...but this is ludicrous.
Hivory Towers is absolutely not one of the games five worlds.
A) The hub has always been treated distinctly in this teams games, and videogames in general. Including the overworld in the count of the games advertised 5 worlds betrays expectations, its sneaky! (These are professionals who are intentional about being clear with this sort of thing)
B) The worlds take place in books, they have resident "ghost writers" and respective transformations. Hivory Towers does not seem to meet these qualifications.
C) Other screens from the source of the image you share seem to show 6-7+ worlds worth of collectibles, devs imply per Twitter this info may be data/inaccurate.
D) You mention 100 Paiges, 1000 Quills seems a nice number pairing but that's pretty subjective to you, and Mario/Banjo didn't have those precise numbers and weren't even decagonal. (e.g. 242 Stars in Mario Galaxy)
E) Anyhoo (don't judge!) ANYHOO, what even is a world in YL, anyway? They grow, they expand, they change! What if, upon expansion, Glacier World melts, or cellabrates Kwanzaa? We'd have a different theme within the same world. We don't know the scale of this games content, so not even clear we even have just 1-2 unknown world themes.
p.s. Really cool about the cone/shopping cart. I think it's just an Easter Egg by Dean who likes all thing d'art. But could be a unique world theme as you say.