Just a technicality really, but this is just an encoding of language — like Morse code or writing — it's not a language of its own.
No, this a language. You may be thinking of say written language which is an encoding of language sounds
You could also call it a register (as the article does) and group it with shouting and whispering, I don't completely agree with that assertion but either way it's most certainly not a language on its own, it's a way of speaking Greek.
> written language which is an encoding of language sounds
That is absolutely false. Written language is _not_ an encoding of language sounds. The International Phonetic Alphabet (IPA) is, but there's a pretty long way from written language to IPA. Written language is a beast of its own and it's lagging way behind spoken language, but it's not an encoding of language sounds. It's a way of encoding language, but phonetics and phonology has nothing to do with it.