I don’t think this is the right takeaway - the examples all had marketing as the second skill because the author wanted them to be people that the reader would

I don’t think this is the right takeaway - the examples all had marketing as the second skill because the author wanted them to be people that the reader would recognize. This means that they needed people with a public persona, which lends to self marketing being the shared skill. A person with a skill set that doesn’t include marketing could be just as successful, but you as a reader aren’t going to connect to the example of the author’s second cousin Sam.