Is there evidence that LLMs adhere to this format better than to JSON? I doubt that.

It is 100% guaranteed that they DON'T. Toon is 3 months old, it's not used by anyone, and it's therefore not in the training set of any model.

Their benchmarks compare it against other formats as input, not as output.

Now it makes sense.

Nice, it would be good idea to develop CFG for this as well so can embed it into all these constrained decoding libraries