I disagree that rating is the sole determinant as to whether one can be a competent coach. I also disagree that it is not possible to be an effective coach for someone having a higher rating. Indeed, in many sports (e.g., tennis), coaches of the very top players cannot beat their students. However, this does not mean they cannot help their students improve. Often, the best coaches were not the best players at the sport or game that they coach. Being able to evaluate a student's weaknesses, understanding how to develop training regimens personalized for particular students and communicating/explaining concepts well are attributes of a good coach/teacher. Top players do not necessarily have these traits.
This is true at the higher levels, but not the lower ones. Of course Carlsen, Anand, Kramnik etc. are going to have seconds (coaches) that do not play as well as they do. However these seconds have the fundamentals mastered, have strengths, and have experience. People in decline make good coaches. They don't compete at as high a level anymore because of time, or aging or whatever but they know what it is like. There is a huge difference between having a lower rated GM or master help you analyse or go over your openings, and a class D patzer passing on his bad habits.
and he also says that he beat GM Nigel Short
im sure that is Short of the truth