The study you're referencing doesn't make that conclusion.

It concludes theres a learning curve that generally takes about 50 hours of time to figure out. The data shows that the one engineer who had more than 50 hours of experience with Cursor actually worked faster.

This is largely my experience, now. I was much slower initially, but I've now figured out the correct way to prompt, guide, and fix the LLM to be effective. I produce way more code and am mentally less fatigued at the end of each day.