I cannot speak for Sophronius of course, but here is one possible answer. It may be that morality is "objective" in the sense that Eliezer tried to defend in the metaethics sequence. Roughly, when someone says X is good they mean that X is part of of a loosely defined set of things that make humans flourish, and by virtue of the psychological unity of mankind we can be reasonably confident that this is a more-or-less well-defined set and that if humans were perfectly informed and rational they would end up agreeing about which things are in it, as the CEV proposal assumes.

Then we can confidently say that both Alice and Bob in your example are objectively mistaken (it is completely implausible that CEV is achieved by painting all buildings the color that Alice or Bob happens to like subjectively the most, as opposed to leaving the decision to the free market, or perhaps careful science-based urban planning done by a FAI). We can also confidently say that some real-world expressions of values (e.g. "Heretics should be burned at the stake", which was popular a few hundred years ago) are false. Others are more debatable. In particular, the last two examples in Sophronius' list are cases where I am reasonably confident that his answers are the correct ones, but not as close to 100%-epsilon probability as I am on the examples I gave above.