what Eliezer seems to be arguing is that what we really care about is not (a) that whatever states of affairs our brains are wired to send reward signals in response to be realized, but (b) that we experience peace and love and harmony and discovery and so on.
His motivation for thinking this way is a thought experiment - which might become real in the relatively near future - about what would happen if a superintelligent machine could rewire our brains. If what we really care about is (a), then we shouldn't object if the superintelligent machine rewires our brains to send reward signals only when we are sitting in a jar.
I don't see what plausible reasoning process could lead you to infer this unlikely statement (about motivation, given how many detail would need to be just right for the statement to happen to be true).
Also, even if you forbid modifying definition of human brain, things that initiate high-reward signals in our brains (or that we actually classify as "harmony" or "love") are very far from what we care about, just as whatever a calculator actually computes is not the same kind of consideration as the logically correct answer, even if you use a good calculator and aren't allowed sabotage. There are many reasons (and contexts) for reward in human brain to not be treated as indicative of goodness of a situation.
I don't understand your second paragraph. It sounds like you are agreeing to me, but your tone suggests you think you are disagreeing with me.
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.