I think what he means as music theory is the next step after what this (blatantly AI-written, but passable) article talks about. This is the part that you take for granted, but it certainly is music theory in the same way that Peano's axioms are number theory even though you don't use them explicitly.

The AI style gets boring very quickly, but I liked the explanation of what Western music notation is optimized for. For more information on this, there's a great and long video at https://youtu.be/Eq3bUFgEcb4.

(Edit: I wrote this around the same time as the reply, which more or less confirmed my hunch)