First, EY makes it abundantly clear that two agents can have a fundamental disagreement on values– it's just not the best (or most helpful) assumption when you're talking about two sane human beings with a vast sea of common frameworks and heuristics.
Secondly, I'm worried about what you're trying to do with words when you suggest we "take them to be unreasonable to intrinsically desire the eating of babies".
If you're making an empirical claim that an alien with fundamentally different terminal values will (say) be uninterested in negotiating mutually beneficial deals, or will make patently suboptimal decisions by its own criteria, or exhibit some other characteristic of what we mean by "unreasonable", then you'd need some strong evidence for that claim.
If instead you openly redefine "reasonable" to include "shares our fundamental moral standards", then the property
it is essential to the concept of morality that it involves shared standards common to all fully reasonable agents
becomes a tautology which no longer excludes "meta-semantic subjectivism", as you put it. So I'm puzzled what you mean.
Talking past each other a bit here. Let me try again.
EY makes it abundantly clear that two agents can have a fundamental disagreement on values
EY allows for disagreement in attitude: you might want one thing, while the babyeaters want something different. Of course I'm not charging him with being unable to accommodate this. The objection is instead that he's unable to accommodate disagreement in moral judgment (at the fundamental level). Normativity as mere semantics, and all that.
Your second point rests on a false dichotomy. I'm not making an emp...
In You Provably Can't Trust Yourself, Eliezer tried to figured out why his audience didn't understand his meta-ethics sequence even after they had followed him through philosophy of language and quantum physics. Meta-ethics is my specialty, and I can't figure out what Eliezer's meta-ethical position is. And at least at this point, professionals like Robin Hanson and Toby Ord couldn't figure it out, either.
Part of the problem is that because Eliezer has gotten little value from professional philosophy, he writes about morality in a highly idiosyncratic way, using terms that would require reading hundreds of posts to understand. I might understand Eliezer's meta-ethics better if he would just cough up his positions on standard meta-ethical debates like cognitivism, motivation, the sources of normativity, moral epistemology, and so on. Nick Beckstead recently told me he thinks Eliezer's meta-ethical views are similar to those of Michael Smith, but I'm not seeing it.
If you think you can help me (and others) understand Eliezer's meta-ethical theory, please leave a comment!
Update: This comment by Richard Chappell made sense of Eliezer's meta-ethics for me.