There was one aspect of that which made intuitive sense to me, but which now that I think about it may not have been adequately explained, ever. Eliezer's position seems to be that from some universal reference frame human beings would be viewed as moral relativists. However it is a serious mistake to think that such universal frames exist! So we shouldn't even try to think from a universal frame. From within the confines of a single, specific reference frame, the experience of morality is that of a realist.

EDIT: Put differently, I think Eliezer might agree that there is a metaphorical stone tablet with the rules of morality spelled out - it's encoded in the information patterns of the 3 lbs of grey matter inside your skull. Maybe Eliezer would say that he is a "subjective realist" or something like that. This is strictly different from moral relativism, where choice of morality is more or less arbitrary. As a subjective realist your morality is different than your pebblesorter friend, but it's not arbitrary. You have only limited control over the morality that evolution and culture gifted you.