Earlier quoted context omitted.
How are currencies not commodities?
Currencies don't have any inherent useful value, while commodities do.
1) It's factually untrue. Currencies have both the value of their materials and the art imposed on them. Now it is true, that the primary driver of their price is not that "inherent" value. The primary driver of their price is that they work as stores for value, as a medium of exchange, and that they are fundamentally tied to the governance regime they are associated with. But there are lots of commodities that have the same characteristics. Gold's price is much more dependent on people's perception of it as a value store than on it's usefulness as an input to industrial production or as an artistic element and energy credits are commodities existent only within the governance regime that created them.
2) Even if you don't buy that loosened definition of commodity in general, in the context of an exchange currencies are universally considered commodities because they have several important characteristics 1) there are both spot & derivative markets for them 2) They are completely fungible, dollars made in Philly are identical to ones made in San Fran 3) It is easy (trivial even) to create standardized contracts around their delivery.