Which is the issue with almost all studies and statistics, what it means depends entirely on what you're measuring.
I can program very very fast if I only consider the happy path, hard code everything and don't bother with things like writing tests defining types or worrying about performance under expected scale. It's all much faster right up until the point it isn't - and then it's much slower. Ai isn't quite so obviously bad, but it can still hide short term gains into long term problems which is what studies tend to focus on as the short term doesn't usually require a study to observe.
I think Ai is similar to outsourcing staff to cheeper counties, replacing ingredients with cheaper alternatives and other MBA style ideas. It's almost always instantly beneficial, but the long term issues are harder to predict, and can have far more varied outcomes dependent on weird specifics of the business.