Earlier quoted context omitted.
Your mistake in turn is to assume that it's impossible for people to judge their own abilities. Lots of people think they could write a decent novel. Most are wrong. Suppose only .01% actually could. If your argument were correct, JK Rowling should assume her chances of writing a decent novel are .01%. She feels fairly confident that she could, but she has to discount that, because people are often mistaken about suc…
That is not an example of JK Rowling judging her own abilities. Her abilities were judged by the market when she published her books. It's possible to know one is good at something, it's just not possible to judge your own abilities .
Her previous books should give her confidence that she'll be able to write another. That is one way in which she can judge her own abilities.