Only one thing I would change: The article mentions the term "syntax" very often, but I don't it is the best choice. When I read syntax, I think about something human-readable, e.g. about programming language syntax. What is really meant is "wire-level encoding" or "serialization format". I think [wire-level] encoding might be the most common used term for protocols.
So, for example, ASN.1 defines syntax for expressing schemas (ITU-T x.680 and others in the x.68x series), and also encoding rules (x.690 and others in the x.69x series).
XDR is one standard for both, syntax and encoding.
In HTTP land syntax is used to refer to encoding. In HTTP/1.x, because the protocol is textual and specified in ABNF, syntax can refer to both things equally well. In HTTP/{2,3} this is no longer the case but it doesn't matter, we continue to use "syntax" to refer to the encoding.