I doubt Derek really believes this. At least for me it would be paralyzing to think that I was below average at everything . I assume what he really means is, "below average in x , but I make up for that with y ." I think it'd be hard to have done as well as Derek if you honestly managed to convince yourself that you were below average intelligence, below average skill in your profession, below average ability to mot…
It's actually the opposite of what you're talking about here; Derek is not below-average capacity, he just has a lot of capacity yet to be realized and is willing to both admit that and take steps to improve.