It's a shame they couldn't use yaml, instead. I compared them and yaml uses about 20% fewer tokens. However, I can understand accuracy, derived from frequency, being more important than token budget.
You can compare JSON and YAML on https://platform.openai.com/tokenizer
"{" + $someKey + ":" + $someValue + "}"
produces a valid JSON. It does - sometimes - and then it's indeed easier to work with. It'll also blow up in your face. Using JSON the right way - via a proper parser and serializer - should be identical to using YAML or any other equivalent format.