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.