That is what we call 'evidence', my guy.
Anyway, we aren't detectives, we make do with what we've got, and right now, "Tropical" and "Arid" together point to the Koppen map. If "Arid" was "Desert", then the theory would be weaker, because it would point to a continuation of in-game biomes, but the choice of "Arid" for the pack's title is conspicuous given that it follows "Tropical". This is literally the strongest theory I've actually ever encountered for predicting DLC since the game launch.
Doesn't mean it's assured, nothing ever is, but even so. I'd bet on it, if there was a pool.