While HTML is not a network protocol, it is still a data format that is standardized and to which many implementations need to conform -- a "protocol" in layman's terms.

Is XMPP a protocol? Yeah, it specifies a machine-machine communication system but it also specifies the structure and formatting of the messages it sends.

Where do you draw the line? If it were a technical discussion, you'd be right that HTML isn't a "protocol". From TFA, I got the sense that they didn't mean a network protocol but something more like a "standard" (without the nasty connotations of that term).