For me, this ''mean of exchange" means daily commercial payment method. NXT still can be exchanged with the "real coins" (mean of exchange), as the relationship between gold and USD. But how are you able to make the exchange rate between NXT and the "real coins" constant? If the demand for the real coins becomes high, you can issue more real coins to keep the rate constant. But if the demand becomes low, you can't buy back them because there is no central bank. Do you really need to keep it constant?