You're missing the point though, which is that that LLM was already trained on trillions of examples of human languages. Once it already knows many languages, inventing one more isn't hard. (Though I don't think it's actually "invented" this language to any great depth.)

Let's see it invent language from scratch without being presented any examples of languages to use as training data. That's what humanity did. Can LLMs do it? I doubt it, not yet.