Yes, I agree that DMs have a responsibility to know the experience of those in their care. Again, they only have the info given to them that is provided by the divers. Should these divers in question have been on the dive, no. But the office running interference and providing info to DM for the following day does not always have the truth. Obviously, lying divers who lie about their experience can and have caused a number of problems in the past and will do so in the future. Those who do it can put themselves in danger as well as those they dive with. That is going to a bit of the extreme but when your dealing especially with advanced dives, people can get into serious trouble. When divers show up at the boat in the morning 5 minutes prior to departure how is a DM supposed to have adequate knowledge of the divers actual experience to communicate to the office staff? I am being frank with that question and not a SA .lol. A DM cant make that call by looking at the divers or their gear. Talking to someone for 5 minutes can provide data but not everything. Obviously not all protocols are infallible. Thats my point.
Of course, I wasn't there and I don't have any firsthand knowledge of what you encountered, but in my experience, most Cozumel dive ops won't take your word (or your cert card or your log book) for it that you are ready for Punta Sur or Barracuda if they don't know you. As you say, divers sometimes will be, shall we say, optimistic in their self-assessment. As Christi says, they will generally have to dive with you for a day or two before they will take you somewhere like that.