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YAML 1.2.2 is also not a JSON superset, as keys are required to be unique:

> The content of a mapping node is an unordered set of key/value node pairs, with the restriction that each of the keys is unique

https://yaml.org/spec/1.2.2/#representation-graph

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YAML says keys MUST be unique, while JSON says they SHOULD be unique. It is therefore possible to produce technically valid JSON documents that aren't valid YAML, so it is in that sense not a strict superset. At the same time those documents aren't portable across JSON implementations either. Rejecting duplicate keys is a valid implementation of the JSON spec.

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