fundamentally, "linguistic illegibility" is a new term for something that we've known about for about a decade now. In RL the more general ideas is "reward hacking" and in NLP it has been called "semantic drift".

I dislike this term because it doesn't explain where this "illegibility" is coming from. Models are post-trained towards non-linguistic goals with (mostly) non-linguistic rewards. A model's reasoning chain is reinforced if it leads to a correct answer or agentic goal. It doesn't need to be linguistically accurate and meanings can drift over training.