You probably misunderstood: gp won't make mistakes in a medical field because he's not a medical professional, so he won't do anything in the medical field.

The same sometimes applies to designers and developers: if developers don't design, then of course they won't make bad designs.

The gist of it all is: don't ridicule people for mistakes, even if they're in their supposed field of expertise. Just let them know they've made a mistake if you see one.

I see ggp point as a beatiful love-speech in a context that is completely different from what was discussed.