He also adds that it easier to do this (combining two or more skills) than it is to be the very best at one thing e.g. an NBA player--it is actually easier, statistically, to become a billionaire than it is to become an NBA player.
There are more than 2,000 billionaires worldwide but only about 480 NBA players (32 teams with roughly 15 people on the roster). And if you think about it, becoming a billionaire usually involves multiple skills (good at programming + business, bill gates. Good at research/analysis + cultivating a rare temperament that allows you to act on the insights produced by the research even when everyone around you is losing their shirt, Warren Buffett etc, etc).
Being an NBA player requires one skill at a minimum. Being tall!