That can be true without being demonstration that we're seeing unbounded progress in language use and understanding. A theory of the limit of NLP is still a theory of NLP and by no means better than other theories.
Umm, you just said it yourself, understanding. The model has no clue whatsoever what any of the output means, just that it scores good.
I don't understand Japanese. But given enough time and feedback I can try all the possible combinations of sounds I can come up with and answer a question in Japanese till I get a satisfying feedback from the Japanese speaker (ex: his facial expression) that asked the question. Is this satisfactory as for speaking Japanese in your opinion?
It's always seemed obvious to me (as an outsider) that it's missing reason and explainability. GPT-3 is a neat tool, but it seems like anthropomorphism to suggest that it's more than the best mimic humanity has been able to create so far.
So a theoretical argument could useful if gave us an idea what are our mysterious something it. However, I'm not sure if this article makes a contribution here. Proofs of impossibility generally aren't useful, as you say.