I make no assumption of correlation between productivity and hours worked in my writing. I just say that increased productivity leaves room for this change.

If productivity is sublinear, that's an even better argument for shortening the week. If you reduce hours worked by x% and production is reduced by much less than x%, that's great for society too. I mean, what's our objective-- chaining people to the workplace or getting things done?

Many jobs requiring coverage / a person to be present will still make the "reduced supply" argument true.