Not sure that “showing how money has been used as a system of debt and credit tracking” is an honest description of currency as a medium of exchange, though it does serve the proposer’s of MMT larger ideological/analytical purposes. The Treasury has over a trillion dollars in $100 bills in circulation, not a lot of credit tracking involved in their use as a medium of exchange. To say that currency “has been used to facilitate transactions” suggests that this is not its primary function, its raison d’etre.