I think you are misrepresenting (or perhaps, misunderstanding) the conclusion of these studies. The increased "free time" is most entirely due to high unemployment at the lower end of income.
If you control for unemployment and under-employment, the graphs pretty much flatten out (as you can observe in the later graphs of the publication you linked below)
Edit: it’s absolutely not true universally and it’s ridiculous to suggest it is. Comparing averages will be very tricky as well.