I don't understand his argument here. He seems to be arguing against the idea that it's actually impossible to get anything productive done after you've worked a certain number of hours rather than the idea that a company or workforce can be more effective if they work fewer hours.

I can work for 100 hours in a week and get strictly more done than I did in the first 40, and yet I wouldn't be surprised if a company that required 100-hour weeks from its employees became more effective when they reduced their work week to 40 hours.