The title is a tad misleading because the HTML in the link is described "an extension to HTML" featuring numbers, booleans and variables.

The popular YouTube channel Computerphile tackled this topic back in 2016:

HTML IS a Programming Language (Imperative vs Declarative): https://www.youtube.com/watch?v=4A2mWqLUpzw

Me? I'm happy to describe it as a markup language.

An interesting related question: Is CSS a style sheet language, or now something more? It has variables, performs calculations (using the calc function), can add strings to the page (using a pseudo element) and can create animation. It might not count as a programming language, but it has morphed into a strange hybrid beast between style sheet and programming language.