JSX stands for JSX. Your definition is something that people just imagine to be true. The React docs do not mention the word XML at all. The “templating” syntax is not XML. It has no defined semantics and does not generally support crucial XML features like namespaces.
You could easily generate an UI based on just the DTD and Schema that could be used to fill a perfectly valid XML file.
Validating incoming XML was a breeze, just give it to the validator class along with the DTD and Schema and boom, done.