That line of defense is drawn by the employer,
Exacty.
Does it really matter what course they took? If an dive operation hired a DM, AND they performed poorly, chances are they are out looking for work. OTOH, if they turn out to be a very good DM, chances are they are pretty bright and learn quickly. In addition, once hired, they are making several dives a day, learning all the time and after a year, they might be one of the top DM's around. Who knows?
I just hate dismissing things out of hand.