Thats a weird advice, most accomplished sportsmen have had coaches less accomplished then them. All top tennis pros, Federer, Nadal and Djokovic have had coaches less accomplished then them. Most football coaches are less accomplished then their star players. I am pretty sure this is true for most fields, not just sports.
Athletes on that level rarely have just one coach throughout their careers, and are constantly striving to do better. Players of team sports demand trades due to (perceived, if not actual) deficiencies in coaching all the time. It's possible, maybe even desirable sometimes, to have someone who is less accomplished than you to coach or guide you, but it would be probably be detrimental to follow everything they said without question.
As I implied in another comment of mine, I agree with you that it's interesting to look at how athletes use coaching. Things like music training are another area with, I think, similar patterns.
Almost by definition, truly top athletes are unlikely to be able to have coaches more accomplished than them, especially when you consider the overall trend-line in sports performance is upward. So clearly "more accomplished than you" is not a strict requirement. But it might still be the optimum for everyone else.
I think my personal model is this: First of all, the most-important asset for a coach is that they have a correct model of improvement in the field. That doesn't strictly require them to be any good at all. However, they're much more likely to develop a correct model if they put a bunch of time into trying to be good. And also, if they were never good, they're much more in danger of having a subtly-totally-wrong model of how improvement would work. So although it's not strictly necessary that they be any good at the activity itself, it markedly improves both upside risk and downside risk.
