He posits that our present assumptions about money (first there's barter, then coins and fiat money, then credit) is actually backwards. And that primitive societies rather had interlocking systems of informal credit between everyone instead, since everyone knew everyone and you only really needed to settle the difference very infrequently.
And coins only really came into play as a supply logistics mechanism for traveling armies. Since a band of people traveling through town can't participate in the informal credit system. And so the taxation system created in such a way that the food producers needed to acquire an "external" currency the army had. And that this was shown archeologically where concentration of coins were denser around militarized border regions of various societies around the world.