In the sense of the values awarded for cooperation vs competition? Sure it's an approximation but that doesn't mean it's arbitrary. The entire point is to explore the nature of altruistic behavior, which we know exists. We know there are deer who groom each other even though it is in each deer's best interest to be groomed but not groom in turn. There is a larger benefit to betrayal than to cooperation but a cost associated with everyone acting selfishly.

The prisoner's dilemma is a model of reality. Sure you can insert numbers that make it work in reverse but it's as valid as saying gravity is 4m/s² proves that I won't die by jumping off this building.