HTML is an application of SGML / ISO8879. It defines its own domain and that's the scope of it. It's a leaf.

XML is a subset of SGML and can have further subsets and implementing applications I.e. it is a branch. As noted above, XHTML was a leaf on this branch.

So they are related but not equivalent.

It's not, technically. XHTML was a formally defined HTML subset of XML but it died.