Right, this looks like horsecrap.
* No randomization. The treatment groups were selected on the basis of existing "high levels of stress". The control groups weren't. So, just regression to the mean could explain any results.
* Everyone wanted in. "The trial grew almost thirty times in size over the next five years, to around 2,500 participating staff in response to early positive results." Sure, who wouldn't demand a shorter week for the same pay? But now you've lost control of your experiment.
* Hello, Hawthorne effect. "Hey, we're testing low hours and if you hit all these KPIs we'll make them permanent!" Solve for the equilibrium.
This isn't science. It's marketing bullshit. Honestly, you can tell that just from the elegantly selected fonts.
Suckers everywhere are going to lap it up. Of course! They want to believe.
As people get richer they want more leisure. That's fine. Any nation can choose to be lotus-eaters. Just don't pretend to have scientifically proved you won't get poorer.