HTML is an initialism for HyperText Markup Language. It's purpose is to provide structure to data and cannot perform logic, the defining feature of programming languages.
I guarantee you that the people writing that HTML isn’t a language are the junior devs that use DIVs for everything in their structure, especially actionable elements like buttons or anchors. Let’s ignore that the “DIV is the element of last resort”, as per the html spec. Also, I doubt that anyone suggesting CSS isn’t a language hasn’t picked up a OReilley’s manual on it recently - 1100+ pages, before you get to the glossary. HTML & CSS are quite a powerful combination.
Magic the Gathering is Turing complete but nobody's calling it a programming language. Expressiveness and purpose are probably more important for calling something a programming language.
I did not think this was a hotly debated topic but, if you're going to call HTML a programming language then you probably aren't far from calling CSS a programming language and that is not a reality I'm willing to accept.