I would consider computation, i.e. transformation of data, as the essence of of a programming language. And the only form of computation, if you will, that HTML is capabale of, is producing constant functions, which, given arbitrary input, will yield a static data structure - albeit one that can certainly be quite complex and very useful.
That being said, that a formal language doesn't constitute a programming language, diminishes its value and importance in absolutely no way. A software system consists of many different artifacts, of which HTML often is an one. And by the way, I also count requirement specifications and user documentation to artifacts that are vital part of a software system. And those are both, at least in most cases, artifacts that, in contrast to HTML, neither follow a formal grammar nor are executable. Computational capabilities the only property that makes something a useful or important.
To conclude, although I disagreeing with the proposed definition for programming language, which is premise of your argument, I indeed very much appreciate the articles sentiment of speaking out against gatekeeping in software development, which in the process of creating a working system isn't useful at all.
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