This is probably due to differences in the schedules of "makers" vs. "managers" [1]. If you're a one-man operation (which I believe the author is), this approach is easy. If you're an individual contributor within a team, it's a bit harder. If you're a team lead or a manager, it's even harder. I'm an engineering manager and if I try to timebox a certain type of task into a specific day of the week, my teams are likely to get blocked until that day arrives.

[1] http://www.paulgraham.com/makersschedule.html