Many musicians give private lessons on their instrument having no education at all on "how to teach". They get their students based on their playing ability and word of mouth.
You also don't hear about people dying or getting hurt because they didn't learn how to play musical instruments properly from an unqualified music teacher. I haven't.
If an "advanced" diver knows his/her skills well and observes safety standards (be logical with conditions, etc.), it is probably fine for them to show people the DSD skills.
There is a lot of "if's" in your statement. Who is to decide and monitor that the person is doing it correctly? And if there is an injury, even a very insignificant injury, do you know how much liability exposure there is to the person doing the teaching without being qualified and properly certified??
To give you an example: I am qualified to teach the full range of recreational SCUBA diving courses but I am only authorized to teach the beginner level of free diving, down to 12 meters. I can probably teach the next course in free diving, down to 20 meters but I am not authorized/certified to teach the second level. I'd NEVER even think of teaching this second course before I am certified to teach it. Not even to a spouse or a child even when I am very experienced and certified instructor in SCUBA and the beginner level in free diving. Contrast this to somebody who is NOT certified/authorized to teach ANYTHING at all, NOTHING!!