Feynman didn't think he was a genius, he said that he was just an ordinary person who worked very hard. The story in the article actually points this out; he was just curious which led to very beneficial working methods. The idea of geniuses is more of an excuse than an explanation.
You can't just start calling people who have a few years of math and physics training geniuses... Infinite sums is a very simple concept once you have some math background and Stokes theorem isn't that complicated either (once you have the right training).
