It's not that different though. At times there are dollars which have become not fungible. We don't say this makes the whole system not fungible. Because in most transactions, most of the time, people don't distinguish between the units of exchange.
In some highly technical way, is the medium not fungible? Sure. Just as in prison exchange of cigarettes there might be someone who won't take Parliaments and someone else who wants only Marbs. But most of the time, most people, exchanging most coin, consider them interchangable. And in any financial tool that is what matters. Not theory but how people treat it.
