This was my guess based on having recently read "The Rise and Fall of American Growth" when this study first dropped. [1] The argument he makes in the book is that a big part of the productivity jump we saw in the middle of the last century wasn't entirely due to the stimulus of WWII as most people assume, but was a synergy of that plus the reduction of workplace injuries, etc, related to the 40 hour work week.
Was frustrated at the time that they seemed to be jumping to conclusions so quickly regarding something that could be transformative to society.