Ugh, this phrase always irritates me. It's one of those throwaway phrases that people just assume has merit without ever critically examining it. It's a paraphrase of Shaw quote who was playing around with something Aristotle said.
It's on par with "Absence of evidence is not evidence of absence." It sounds good and all until you dig a bit deeper and realize it's just a bunch of pretentious nonsense.
Maybe, just maybe, teaching is harder than simply doing, which is why doers can't necessarily teach.
Are you a teacher by any chance?
The phrase is obviously rooted in reality. You'll be hard pressed to find a hockey player in his prime whose dream or passion is to coach rather than play and quits his playing career to teach. That extends to other areas/disciplines.
I won't venture into whether teaching is harder. At the end of the day you're dealing with individuals. When it comes to athletes, there are things you can't teach no matter how good the coach is.
But more on topic, it's hard to evaluate coaching without experiencing it in person. Krog was no doubt a skilled player but if he's skilled because of natural talent combined with hard work, how teachable is that? The same with Hockey IQ. A player can have high hockey IQ as a player but instincts aren't exactly teachable. A player who was slow to process the game might actually have a good hockey mind for teaching. Maybe Krog worked on his weaknesses a lot.
It's really another topic altogether. Krog has been teaching his son so it's possible that he's actually put in a lot of time in learning how to develop hockey players. At the same time, a lot of it is respect, communications, and personality. The trend these days is to hire ex-NHL players who were great NHL players and there are tricks of the trade.