That's true, but in terms of 'fully free' days it does apply.

For example, I'd bet most people would be unwilling to smooth over their 40 hour workweek over the course of the full week of 7 days, working 5.7 hours a day. Of course that's nice in a way, but having no fully free days and perpetually working forever, is agonising.

In that sense you can't just compare cutting a working day to simply having more discretionary time (which you have after workdays, too). Fully free days count a bit differently.

True! Though on work days you do need to wind down and possibly commute. You also spend one less night possibly mentally preparing for the next work day. While not 100% of the working, work days certainly can feel overall very different in the downtime than days off.

And you don't spend 100% of your days off not working.

The worker interviewed in the article said, "Also being on call can be a challenge, but it's nothing that's not fixable."

I understand that to mean that he's not truly working a 4-day week. He's working 4 days + on-call day(s) at times.

We operate a five-nines service in a 4-day work week. That requires at least one person always being connected.

But culturally, it also encourages a lot of automation, redundancy, and conservative DevOps decisions.