What I'd really want to see is a histogram of weekly hours worked per worker for each country
For starters, it shows time spent at work. Meanwhile employees can do varying amount of work in the same amount of time. And I suppose that's what those Germans you referring to mean.
Second as the document notes: "The results are affected by the varying proportions of part-time workers across countries, in addition to differences in legal frameworks and in country-specific usual length of the workweek".
Eventually he cut his working hours in half, while actually doubling his output, because the shorter work hours required him to actually focus.
He was, of course, self-employed, and could design his work week how he liked.
I guess that's important for another reason: if someone else had been paying him by the hour, he would have experienced a 50% pay cut. Instead, his income doubled, because it was based on the actual results.