I would expect any change to an optimized workflow (developing own well understood project) to initially be slower. What I'd like to see is how these same developers do 6 months or a year from now after using AI has become the natural workflow on these same projects. The article mentions that these results don't extrapolate to other devs, but it's important to note that it may not extrapolate over time to these same devs.
I myself am just getting started and I can see how so many things can be scripted with AI that would be very difficult to (semi-)automate without. You gotta ask yourself "Is it worth the time?"[0]