"... prior LLM usage is negatively and significantly correlated with written communication and vibe coding performance. We speculate that this relationship is caused either by (i) LLMs having a negative effect on students’ ability to express themselves or (ii) that students who are less proficient at writing are more likely to use LLMs,"

Makes sense right? If you let the AI write and express ideas for you you are not really practicing that skill yourself. It is just like asking someone else do your work for you, you won't learn anything about it.