Earlier quoted context omitted.
To paraphrase, it is not that most leadership/high management is poorer than expected, it appears poorer even when it is expected to be poorer than expected. At some point between the ages of 20 and 30, you begin to believe that, although it is difficult to point out what they are better at than you or your peers, in order to be in the position they are in, "leaders" must be better in some way--say, in the technical…
Im convinced that a lot of leaders, especially in tech, are leaders not because they are smart or skilled, but simply because they were there early.
It is also a virtuous circle: you come in early, you are promoted because there is virtually no one else around, or you are chosen as the best among a group of fairly mediocre people--maybe they use a die to choose who will become the new "leader" of the group--and in a couple of months you go from being just another IC to a rising star.
You are quickly recognized as such and, in an environment with very loose feedback cycles such as management in the technology sector, you are free to fake your way up to the top. As soon as you are exposed, if ever, you can recycle yourself elsewhere as a true and proven leader of vast experience.
Or, assuming you are not very good at leadership in absolute terms, you do a good job anyway because most management is skirting responsibilities, taking advantage, in a good way, of colleagues and the machinery that makes the company work, and, most of all, going with the flow.