The question at hand is about the role of a DiveMaster, not the skill level. There is no indication that the DM in question was not certified--in fact, it was likely that they were a full Open Water Instructor, as is common on boat DiveMasters. It is almost certain that this person was experienced and had training in safety procedures. Having such a person along on a dive and tasked with safety and general dive planning increases safety, but it does not mean that everyone is equally safe. Those who deviate from the plan (dive deeper than planned) or leave the area visible to an in-water DiveMaster are electing to be less safe than those who do not. The assumption of that risk is borne by the individual diver. Many divers will elect to assume that risk to be more independent or to have more flexibility in their dive plan.
To extend your allegory, suppose your archaeologist/guide was conducting a tour, pointing out things with the group he was guiding and helping to ensure they didn't damage the ruins or hurt themselves. If someone in the group wanders off on their own, unknown to the guide, and tripped off one of the cliff faces, would you hold the archaeologist responsible for not monitoring every person in the group? If that same archaeologist was overlooking the site from a vantage point and saw someone getting dangerously close to an edge, he could have yelled to warn the individual, but could not have pulled him away from the edge. Both situations involved an equally knowledgeable guide, but each made a judgment about their vantage point and role in the group. Neither could be reasonably expected to provide complete safety to everyone in the group. There's a similar tradeoff between direct in-water supervision (ability to directly assist, but limited perspective on those who elect to go out of range) and surface/boat supervision (less ability to directly assist, but greater perspective).
If a DiveMaster gives an appropriate dive briefing including their role on the dive, acts in a way that is appropriately conservative during the dive, and renders emergency assistance to the best of their ability should the need arise, I believe they completely fill their role as an DiveMaster. They cannot be everywhere at once, and should not be expected to assume all risk and safety responsibility for all divers. If a DiveMaster pushes a student beyond their capabilities with full knowledge, they assume liability for that irresponsible decision. If a DiveMaster is aware of a potential or actual emergency and either does not address it or fails to act properly, they again assume liability for their inaction or ineffective action. But they should not be expected to be aware of every emergency. A dive area can be large, visibility can be limited, and the human field of vision is only 60 degrees, usually less with a mask.