You’re probably aware of this inherent contradiction by for the sake of any third parties reading:
TotallynotJessica is advocating for virtue, contract, and rule based ethical paradigms based on the hypothesis that they will, in general, more effectively lead to outcomes preferred by utilitarianism.
I think this contradiction is only important to people that are entrenched on one side or the other (or the other, or the other). For people that just want to understand how to make good decisions in their lives it’s a bit of a moot point.
The person you asked your question of claims to be a biologist, but you dismiss the relevance of biology.
It sounds like you might be more interested in an answer from a sociologist. Or are you asking the biologist to argue that basing it on biology is not flawed?