The fact that the OP failed in fulfilling his critical responsibilities as a DM AND also can't understand why arm swimming is not the answer and continues to wonder if it was the wetsuit, is ample evidence for someone to conclude that the diver's abilities and training are insufficient for the task at hand.
Someone with adequate training and skills, should be able to recognize the cause of the failure, or at least have a very good idea about what caused it. Anyone can make a mistake and screw up, but with the benefit of adequate skills and training, the probable causes of the mistake should be readily apparent to the individual.
With respect to judging someone's own training, how would they determine if it is/was adequate? I would think that if that person can not identify the CAUSE of their failure that this should be a very clear signal that the training was incomplete. It was not a super complex situation.
The inability to adequately analyze the situation (even with ample help from others) is critical. In other words, the inability to self analyze is much more relevant than the simple occurrence of the "problem" itself.
As has been emphasized earlier in this thread, we are not talking about a recreational diver, we are talking about a fully certified and insured individual who is presumably qualified to assume responsibilities for others.
Perhaps my perspective is too blunt, but I would suggest that the OP try to find some other, independent professional to perfom an independent evaluation of their skills/knowledge/watermanship and then go from there.