Yes, part of the edge does not have corals but what is happening is that divers hook into the actual coral that is there. Or they grab on like in the photo provided by Dennis. On the Aggressor that is actually part of the reef hook briefing. "Look for dead coral." I am sure some divers like you are able to avoid living coral just by looking at it, maneuvering in current to a dead spot and hooking in, but many are not. There are other hook in dive sites in Palau where I worked very, very hard to find something I thought was not alive to hook into.
Dennis your question above is amusing like someone is a proponent of "grabbing the reef." What a ridiculous question. Hmmm. Do you think grabbing the reef or a reef hook is better? Good question.
Once again, if we cannot dive these sites without damaging them in one way or another, we shouldn't be diving them. And, it is not just the reef hook. Any site that requires divers to continually come in contact with them in strong current and surge, will eventually be destroyed. By hands, hooks, fins, bodies, fin wash etc.