[BulgarianIdiot]

You're right, but an LLM is already trying to make sense (i.e. predict well) within the constraints given. So if you constrain the syntax, it's trying to fill it with the correct semantics. Doesn't always manage it, but it's trying.

This is similar to the way where if you ask a question in a given language, it responds in that language. But it still follows the instructions (hidden prompt) that was given to it only in English.

I.e. an LLM is essentially about finding an intersection of requirements in order to predict output.