That’s why it’s a superset ;)

You can map anything to it but that flexibility means you then need to define schemas et al to ensure compliance

The schema thing isn’t actually unique to XML either. you can do the same thing with JSON and YAML too. But in my opinion, if you get into the realm of needing schemas in JSON then you’re at the point where you shouldn’t be using JSON any longer since you’re now violating the “simplicity” argument which is JSONs only real advantage over other formats.

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.