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…
The fixed theory of intelligence versus the growth/incremental.
You're right: no use to be an Eeyore and assume you're doomed.
But if you're dedicated to learning, then assuming you're currently below average keeps you learning harder than if you assume you're most of the way there.