There is some significant problem with moral deference itself that has no analogue in ordinary cases of deference.
I've talked about how there are obvious problems with arguing this -- in order to prove it, you need, for instance, to keep the levels of generality the same, which arguments for moral deference pessimism never do. But let's set all of this aside, and let us ask a question. How could anybody know that DATUM is true?
Usually moral deference pessimists will say it is 'intuitively true', taking cover behind the vagueness of 'intuition' in analytic philosophy. But we can ask, what kind of intuition? Is it a sensory perception? That can't be right. Is it a self-evident principle, known by intellectual insight? It doesn't seem to be. It's not anything like a recognition of causality, and if it were a memory, a result of analogical reasoning, or a result of reasoning about coherence, this would just push the matter back to ask what the starting point was. It's not something children seem to know, and if that's the case it seems it would have to be learned. And what it seems to be is something known by testimony.
Put it a different way. Suppose that by 'intuitively' I meant just 'as it appears to me'. That's not strong enough as a foundation to establish that there really is something amiss with moral deference that is not present in ordinary cases of deference; someone could very well reply that this is because I am looking at it the wrong way, or because I am gullible, or because I am just weird in some way. To get moral deference pessimism you need to take the problem to be something that can be seen when you take moral deference generally. But the only way in which we can have a sense of moral deference generally is by drawing on the testimony of others.
Now, if DATUM were simply about something like the existence of moral deference, it would be a purely factual question, and there would be no problem. But DATUM is not a purely factual claim but a normative one, about morality. Thus the only way DATUM could be known to be true as a moral matter, it seems, is by testimony; this is, in fact, moral deference. Thus moral deference pessimism seems to be something that can only be established by moral deference.
Of course, moral deference pessimists would not give up the fight on the basis of this argument. But no moral deference pessimism can get off the ground without establishing an alternate route to DATUM.
It's worth considering in any case, if we take the moral deference problem and turn it on its head. In every other major matter of human interest, in every other field of great endeavor and high achievement, we take deference, on at least some things, to be a standard component. How does the human race get great achievements, make great discoveries, conceive great ideas, in general? By cooperating. So why would anybody think that everyone can get a robust version of something as valuable as morality simply by relying on what goes on inside their own heads?