For example, he speculates that gluttony may be seen as a sin, because it was a way for disempowered people to feel superiority over people who had an abundance of food and wealth. If this frame is true (a big if), then one can be guided to not care about morality, and do what is in one's best interest.
Of course, there are good psychological reasons for why it is a bad reason to throw away morality. If one's identity is tied to being a good person, doing bad acts in conflict with one's identity can cause a lot of distress. There is also a "hedonistic treadmill" where one gets used to more and more wealth, and how additional wealth at a certain point doesn't make someone happier. Lastly, there are society reasons for why morality is important (e.g. the Golden Rule as you wrote); a low-trust society where no one is moral is not a healthy society to live in.
Counter-arguments aside, there is quite a bit of writing, also by philosophers, who argue that morality does exist for dominance alone (though I disagree with this, largely from a psychological rather than a philosophical perspective, due to lacking background to make rigorous-enough philosophical arguments).