I think what hard money adherents miss is that interest involves a flow of money. Collecting interest is possible even in a fixed-quantity money regime.
The initial loan + interest isn’t handed over in one lump sum, it’s done in increments. There isn’t a need for the quantity of money to expand in order for interest to be paid, which I suppose Plato didn’t get.
All the same, “credit money” as opposed to hard money, is as old as those 13th century banks and trade houses. They began balancing accounts by exchanging and circulating each other’s bills. If you were a trusted house/bank no one bothered to demand gold itself.
/bingo