I just listened to the episode and I am not sure I buy their premise why bad managers exists. There are many reason why someone get promoted into a managerial position, but saying it's because they were good at their current job seems simplistic and a broad generalization.

I am also skeptical of their claim about sales numbers and how effective their managerial skills are. Correlation is not causation, and averages is a horrible way to judge if a person will be a good manager.