interesting paper and lots of really well done bits. As a senior dev that uses LLM extensively: This paper was using copilot in 2023 mostly. I used it and chatgpt in that timeframe, and took chatgpts output 90% of the time; copilot was rarely good beyond very basic boilerplate for me, in that time period. Which might explain why it helped jr devs so much in the study.
Somewhat related, i have a good idea what i can and cannot ask chatgpt for, ie when it will and wont help. That is partially usage related and partially dev experience related. I usually ask it to not generate full examples, only minimal snippets which helps quite a bit.