Notably, looking up graphs of GDP per capita of France and the UK and putting them next to each other, I'd challenge anyone to see when the change happened.
One thing that comparing GDP per capita growth tends to show is that global and regional overall development totally swamps even seemingly large changes like the 35 hour week.
What the change also tends to gloss over is that the number of hours worked per worker in France is not particularly low. According to OECD, for 2019 it stood at 1505 vs. 1538 for the UK or 1386 for Germany. The reduction in France, to the extent it changed anything, may have distributed work a bit more but it did not suddenly drastically reduce the hours worked.
The average number of hours worked per worker dropped somewhat over the few years after the reform, but it dropped across all the major OECD countries, and by similar rates.