Well, in the summary he cites some evidence that "geniusness" can be learned, at least if your teacher himself is a genius. "J. J. Thompson and Ernest Rutherford between them trained seventeen Nobel laureates" - quite a coincidence.
Of course, they were the top people in their fields and so attracted the top students. Having good people coming in helps with a person's production of great people.
Maybe they had just accumulated a good set of heuristics for doing good research.