it doesn't do anything on its own. Html tags are meant to be parsed and used by the underlying programming language that adds the actual functionality. Nobody is saying it's simple, or useless it's just not a programming language. I'd be surprised if it's even Turing complete, but even then it's still about how it's used and right now it's for browsers to create the DOM with C or C++, and then exposes this object to JS.
(I didn't see any real explanation in the article, it's more a of an HTML praise, which I get, I think it has done a lot for the web)