I'm pretty sure Eliezer is actually wrong about whether he's a meta-ethical relativist, mainly because he's using words in a slightly different way from the way they use them. Or rather, he thinks that MER is using one specific word in a way that isn't really kosher. (A statement which I think he's basically correct about, but it's a purely semantic quibble and so a stupid thing to argue about.)

Basically, Eliezer is arguing that when he says something is "good" that's a factual claim with factual content. And he's right; he means something specific-although-hard-to-compute by that sentence. And similarly, when I say something is "good" that's another factual claim with factual content, whose truth is at least in theory computable.

But importantly, when Eliezer says something is "good" he doesn't mean quite the same thing I mean when I say something is "good." We actually speak slightly different languages in which the word "good" has slightly different meanings. Meta-Ethical Relativism, at least as summarized by wikipedia, describes this fact with the sentence "terms such as "good," "bad," "right" and "wrong" do not stand subject to universal truth conditions at all." Eliezer doesn't like that because in each speaker's language, terms like "good" stand subject to universal truth conditions. But each speaker speaks a slightly different language where the truth conditions on the word represented by the string "good" stands subject to a slightly different set of universal truth conditions.

For an analogy: I apparently consistently define "blonde" differently from almost everyone I know. But it has an actual definition. When I call someone "blonde" I know what I mean, and people who know me well know what I mean. But it's a different thing from what almost everyone else means when they say "blonde." (I don't know why I can't fix this; I think my color perception is kinda screwed up). An MER guy would say that whether someone is "blonde" isn't objectively true or false because what it means varies from speaker to speaker. Eliezer would say that "blonde" has a meaning in my language and a different meaning in my friends' language, but in either language whether a person is "blonde" is in fact an objective fact.

And, you know, he's right. But we're not very good at discussing phenomena where two different people speak the same language except one or two words have different meanings; it's actually a thing that's hard to talk about. So in practice, "'good' doesn't have an objective definition" conveys my meaning more accurately to the average listener than "'good' has one objective meaning in my language and a different objective meaning in your language."