I agree with everything you're saying. Two things to add to this:

Rosenberg brings up the Euthyphro Dilemma only to eschew it at the end. Are things good (from our perspective) because of our emotional reaction to them or do we have a positive emotional reaction to them because they are good (or seem good to us)?

Additionally as is often the case with this kind of moral relativist argument there's an implicit turn towards moral realism at the end which contradicts its premises. What authority can his argument have to me if he says we ought not to judge people for holding moral viewpoints which appear abhorrent to us if I accept that the good is simply that which I react positively to? Only if I find the idea of judging people from a non-objective standpoint more abhorrent than I find honor killings could this hold water for me, and I don't think most people would.