Eh, XML is as much of a superset of JSON as the Turing machine is a superset of context-free grammars. The former has all the _power_ of the latter and more, but the mapping between them is non-trivial, far from an embedding.

I think it's fine to say C#'s data model is a superset of Java's or Rust's a superset of C's, but XML and JSON is too far for me, personally.