I conclude that you use "I don't understand" here to indicate that you don't find the reasoning compelling. I don't find it compelling, either -- hence, my not endorsing it -- so I don't have anything more to add on that front.
If those people propose that utility functions are timeless (e.g. the Mathematical Universe), or simply an intrinsic part of the quantum amplitudes that make up physical reality (is there a meaningful difference?), then under that assumption I agree. If beauty can be captured as a logical function then women.beautiful is right independent of any agent that might endorse that function. The problem of differing tastes, differing aesthetic value, that lead to sentences like "beauty is in the eye of the beholder" are a result of trying to derive func...
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.