That's not quite how I remember it. Maybe it was a different source, but the idea is that the manager should spend a significant chunk of their time doing actual technical work, so they retain enough proficiency to step in and substitute for any of their subordinates e.g. in the case of sickness.

Thus requires a deep hierarchy of managers to even out the administrative burden and give everyone time to do technical work.

What confuses me is that in my experience, a deep hierarchy of managers tends to lead to each layer inventing more administrative work for themselves and the layers below, not help take the burden off...