> XML is fundamentally incompatible with commonly used programming data structures, namely lists/arrays and structs/maps.

Another way to say this is XML is a grammar formalism that deals purely with serialisation rather than higher-level structures that might be serialised such as co-inductive data structures.

> While HTML technically is not XML, it's very close to it and XHTML still is a thing.

XML and HTML-as-serialisation-format are both subsets of SGML.