We have to be careful here. Skarsaune is *not* presupposing that pleasure is intrinsically good and that goodness existed prior to any human being. That *is* something he believes, but that’s a claim argued for elsewhere.
What he is asking the reader to do here is to “assume for the sake of argument” that moral realism is true. Can we then come up with a plausible story that shows how even if our moral beliefs are the products of evolution those beliefs would nevertheless be able to *track the truth* about moral goodness.
He’s trying to show how in a moral realist world our beliefs can be the product of evolution and *still* be reliable and capable of converging on moral facts.
The conversation kinda goes like this:
Ruse: If evolution is responsible for our moral beliefs, then our moral beliefs are unjustified because those beliefs probably wouldn’t accurately reflect whatever moral facts actually exist.
Skarsaune: But wait, evolution can be responsible for shaping our moral beliefs, but here’s one mechanism by which our moral beliefs can accurately reflect whatever moral facts actually exist, (insert the quote you mentioned here).
That’s all he’s doing, providing us with a mechanism as to how this could happen.
Now, you’d object by asking “okay, even if I grant that, how do we know that pleasure actually *is* good in order to accept your story?”
And then he’d engage in the type of meta-ethical theorizing we’re all familiar with in support of the view that pleasure is good. But we can’t lose focus of the goal he had in mind in that quote, which is quite different from this latter question.