There's no other source of morality and there's no other criterion to evaluate a behaviour's moral worth by. (Theorised sources such as "God" or "innate human goodness" or "empathy" are incorrect; criteria like "the golden rule" or "the Kantian imperative" or "utility maximisation" are only correct to the extent that they mirror the game theory evaluation.)

Of course we claim to have other sources and we act according to those sources; the claim is that those moral-according-to-X behaviours are immoral.

what is different about how we value morality based on its origin?

Evolution, either genetic or cultural, doesn't have infinite search capacity. We can evaluate which of our adaptations actually are promoting or enforcing symmetric cooperation in the IPD, and which are still climbing that hill, or are harmless extraneous adaptations generated by the search but not yet optimised away by selection pressures.