Working based on time i.e. 5 days a week is already problematic. We all see the pay by the hour workers like pool cleaners, vendor machine stocking people etc spending lots of time dragging out their work as they get paid by the hour. It makes perfect sense from their perspective and yes not everyone drags the work.

Fixing the work week to just 5 days have similar issues. Some weeks will be less work and other weeks more work but you spend the same five days there. So the what you learn that matters is to spend 5 days physically there and perform a minimum workload so you don't get fired. You drag the weeks with less work and pick up inefficient habits as a result. That is what a 5 day working week teaches. Again there will be exceptions.

Now assuming this study is correct I am not surprised with the results. You just incentivized workers to get the same amount of output done with the condition that you gain 1 day off. Off course workers will find better and quicker ways of working to get that day off.

Even if we did a 4 working day week the problem of working based on time either fixed or paid by the hour remains. The incentivisation is the problem.