The article gets it at the end even if the author doesn't appear to: in order to talk about mediocrity on an absolute scale you have to be thinking of achievement on an absolute scale, and that a flawed approach no matter what percentile you are at. Achievement, and happiness, and other metrics of utilitarian outcomes, should be measured in context, against your inclinations and potential and situation. Trying to grade yourself against an average or aggregate scale will not only be disappointing, it won't even help you accomplish the right things for yourself.