Currency is a medium of exchange, nothing more or less. If use cigarettes, Pokemon cards or abstract numbers on a computer to trade for goods, services or abstract numbers, then they have currency. Banging on about debt and whatnot just focuses on one type of currency.
Nobody prevents you to exchange goods, services or dollars for cigarettes, Pokemon cards or abstract numbers on a computer. But this doesn't make cigarettes, Pokemon cards or abstract numbers a currency. A currency is legally enforceable instrument which originated from loan contracts. A cigarette is a small cylinder of finely cut tobacco leaves rolled in thin paper for smoking. A Pokemon card is a collectible card based on the Pokémon video game series. A number is an abstract mathematical object. So, you can exchange whatever you want, but thing A will not magically become thing B because of that.
P.S. I have a number 5 written in my Word document. Are you interested in buying it? The price is just $1,000. On a standard Bitcoin exchange the same number costs $65,000.