Re-read the manifesto. You'll notice that its theme is not about a person's natural abilities or smarts, but self awareness. The manifesto can basically be boiled down to "show humility in your own code". Always assume that there is potential for improvement, and be ready for when that potential becomes a need.

It's about mindfulness towards our own product. "Mediocrity", in this case, is turning a blind eye towards our own limits. All people have the ability to overcome this form of mediocrity.