It's not “just” that, because monetary systems don't work like that at all. The point of GP is that while the myth says “in the beginning there was barter, and

It's not “just” that, because monetary systems don't work like that at all. The point of GP is that while the myth says “in the beginning there was barter, and barter is inefficient so we created money to allow trading more efficiently” the reality is more “In the beginning there was trade, and some traded goods became money because they had the correct properties”.