I was thinking that too, but stand-alone DLC would have to both give you a full map to explore below the caves and keep you from the seeing the overworld. Not sure if they'd do both. Plus, it's being billed as a full sequel.
They mentioned the game was born out of too many ideas for DLC, which I think is cool. Definitely confirms the DLC vibe, in a way. Also hints the game could come as early as next year, with the engine already built, no need to move to a new console, and the team having been working on it since the last DLC.