Discussion in the previous post brought out that there are two very different kinds of disagreement cases, and for at least one of these kinds, actual disagreement is importantly undermining in a way that merely imagined disagreement is not. As Daniel put it: if we learn that my thermometer actually disagrees with yours about the temperature, that's epistemically significant in a way that merely imagining a divergent thermometer reading is not. But I think the sort of "ideal disagreement" that metaethicists are interested in does not share this feature. So let me say a bit more about these two kinds of disagreement, and why the modal status of a disagreement (as actual or merely possible) makes no difference in cases of the second kind.
In cases of what we might call 'non-ideal' disagreement, there's a presumption that the disagreement is rationally resolvable through the identification of some fallacy or procedural mis-step in the reasoning of either ourselves or our interlocutor. The disagreement is 'non-ideal' in the sense that we're only disagreeing because one of us made a blunder somewhere. We are sufficiently similar in our fundamental epistemic standards and methods that we can generally treat the other's output as a sign of what we (when not malfunctioning) would output. The epistemic significance of the disagreement is thus that the conflicting judgment of a previously-reliable source is some evidence that we have made a blunder, by our own lights, though we may not yet have seen it. Now, obviously, merely imagining a blunder-detector going off in our vicinity is no more evidence of an actual blunder on our part than is an imagined fire siren evidence of an actual fire.
The case of 'ideal' or irresolvable disagreement is rather different. In this case, both agents are (we may stipulate) logically omniscient and hence fully confident that they have not made any procedural blunder in their reasoning. The other's disagreement casts no doubt on this, because the disagreement is instead traceable to a much more fundamental disagreement (perhaps about our epistemic priors, or about whether the moral badness of pain waxes and wanes with the moon).
Here the epistemic significance of the disagreement is more indirect. It's significant just in that it brings to our attention a fact that we might otherwise have neglected: there's this internally coherent alternative worldview against which we can muster no non-question-begging argument. But of course we might just as well be gripped and troubled by this fact even if there never actually existed any advocate for the view in question. The challenge is just: why accept our worldview rather than some other? Any answer we try to give will naturally draw on the assumptions of our own worldview, and hence prove dialectically unsatisfying. But given the inevitability of this, as I keep insisting, it shouldn't bother us too much.