XML doesn’t support booleans. XML Schema, one of several separate add-on schema languages for XML does, but comparing XML + XML Schema to bare JSON does not make sense.

XML Schema isn't only for validating XML post-hoc, as you suggest. The point is that if you are using XML, it doesn't make sense to invent arbitrary solutions when there is already first class support in standardized toolchains. Comparing the markup specs, specifically, is an unreasonably narrow comparison. The discussion is about "using XML" and "using JSON", and the environments in which they're used are at least as important as the specs, independently.

If you are somehow in an environment where you are working with data nominally encoded according to XML spec, yet unable to leverage any other parts of the standard XML ecosystem, then I would certainly agree that XML is a bad choice!