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The calculation was back of the napkin of course, and worst case scenario. But it never implied all people and professions are the same (that's not how statistics work), or that weekly productivity varies with hours worked. Rather with days worked. A day is a fully self-contained unit of work/life so to speak: it contains the commute, the breaks, the work, the recovery time (sleep), etc. This makes it a much more relevant unit of measure.

8 hour days are probably a good balance between how much work you can productively put in during one day and the auxiliary time. Increasing the number of hours per day would probably decrease productivity via fatigue. Decreasing the hours wouldn't bring much of a benefit for the worker since the auxiliary time stays mostly the same (you still commute).

A good chunk of the lost productivity would probably be offset by higher worker satisfaction.