> it eliminates entire swaths of syntactical errors, allowing the LLM to focus on the semantics

As I understood it, the idea the article presents is basically the opposite: The LLM is completely clueless about the semantics, but by constraining the language so that semantic errors are syntax errors, we can catch the LLM's bullshit with a simple validation step and make it try again until it produces valid syntax, which limits how wrong it can get the semantics.

It does apply to JSON in a way: An LLM doesn't always produce valid JSON, but it's easy to do a syntax check with a JSON parser and make the LLM try again until the JSON is valid. The JSON might still be semantically wrong on the level of the application domain (have the wrong values), but at least you can be sure that the generated data consists of lists, dictionaries, strings and numbers.