In what sense is it unaccounted for?
Heck, he hasnt even defined "complexity" yet.
We don't have any known examples of mutations which increase complexity. We know of some mutations which are beneficial, but not that increase complexity (which is required to go from single-celled organisms to people). Most mutations are destructive.
We know of many different operations which cause change-over-time for organisms, but all of them except mutation involve genes already in the environment, and we have no evidence that mutation has ever increased complexity within an organism.