The only overlap they really gave in the examples was "They're good at X skill.... and marketing themselves." Literally, the example people were good at both a skill and then marketing themselves as being good at that skill. Gary Vaynerchuk is both a good writer and good at marketing himself. Steve Jobs was good at design and also marketing himself. Tomas Pueyo is apparently decent at engineering and also ... marketing himself, with articles and TED talks like this. This is also why people can name Neil deGrasse Tyson but probably not a single other living astrophysicist. He's not the best in the world, but he's the best astrophysicist in the world at marketing himself.
So the advice isn't really to be good at two different skills. The advice is to be pretty good at one skill and then be really good at marketing yourself as being good at that thing. Even though other people in the world will be better at it, if you're good at marketing yourself you'll still be in more demand.