Why Being a Jack of All Trades Is Better

That makes no sense. Experts aren't limited to being knowledgeable about one specific topic. It's possible to be an expert on one topic and "just good enough" in many other subjects.

(Compare Stephen Jay Gould, who was a leading evolutionary biologist and writer of popular science, with broad non-expert interests ranging from baseball and Gilbert and Sullivan operas to the size of chocolate bars, to Paul Dirac, who was a leading physicist with almost no interests outside of physics.)

"Steve Jobs is not a better marketer than any of the best marketers in the world."

Being an "expert" at something doesn't mean that one is the best in the world. Quoting Wikitionary, an expert is "a person with extensive knowledge or ability in a given subject." This is also why we use the term "top expert" to distinguish between someone who is an expert, and the best of those who are experts.

If Jobs is as competent at marketing as the best marketers in the world, that makes Jobs an expert marketer almost by definition.

"Barack Obama is not a better lawyer than, say, the Attorney General"

Obama was a constitutional law professor at the University of Chicago. I find it hard to believe that U. Chicago would hire non-experts as professors. While I also find it hard to believe that Loretta Lynch, the current Attorney General, would have been hired as a professor of constitutional law given her background. These are different skills.