One interpretation of CEV is that it's supposed to find these implicit preferences, assuming that everyone has the same, or "similar enough", implicit preferences.
Where does the "everyone" come in? Your initial statement of EY;s metaethics is that it is about my preferences, hoever implicit or extrapolated. Are individual's extrapolated preferences supposed to converge or not? That's a very important issue. If they do converge, then why the emphasis on the difference between should_Peter and should_Matt? If they don't converge, how do you avoid Prudent Predation. The whle thing's as clear as mud.
Where does the "everyone" come in?
One part of EY's theory is that all humans have similar enough implicit preferences that you can talk about implicit human preferences. CEV is supposed to find implicit human preferences.
Others have noted that there's no reason why you can't run CEV on other groups, or a single person, or perhaps only part of a single person. In which case, you can think of CEV(X) as a function that returns the implicit preferences of X, if they exist. This probably accounts for the ambiguity.
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.