Yes, and furthermore, what does Phil mean by "evolutionarily stable"? I'm not asking for the definition of an evolutionarily stable state but rather an explanation for what Phil means by it in this context.

I think Phil may be saying that persistent moral systems must be evolutionary stable, though that raises the question why the moral system needs to be persistent. One might argue that a species that can't support its existence in a moral way should accept its own extinction (that is, the individual members of the species should accept the extinction of the whole species), along with the moral system that led to that conclusion.

Because it won't last if it isn't. If you propose a moral system, knowing that the inevitable consequences of people adopting this system is that they will be exploited by defectors and the system will collapse, leaving an immoral and low-utility society of defectors, that's not moral.