Yes, Forbes "misses the point" of the "4-day workweek" as preached by 37-signals... but there's a good reason for that: they weren't talking about that version of the 4-day workweek.
The Forbes article clearly refers to a 4-day-workweek scheme instituted by Chrysler, not 37-signals - and their analysis of why it's a load of bollocks is pretty much spot on. Forbes hasn't missed the point - Chrysler has.. assuming they were even aware of the existence of that point in the first place, which I doubt.
Here we have a case of two things called by the same terms but referring to very different initiatives.
Ironically, I think 37-signals missed the point that the article wasn't about them!