It's only a valid point in this context, where you ascribe currency the attributes of language.

In real world, currency is only a commodity, it is not currency. By that statement I mean that currency is money, and money is a measure of value/a system to measure value with, but a measure or a metric is not what money is anywhere, money is only a commodity just like currency, and not a metric or system, it has no standards, the units are subject to supply and demand(wrap your head around that), a metric is an abstraction that is used in the real world to measure, as money is by definition, but by utility, actually it is only a commodity. So when the statement “Oh Bitcoin, that’s useless, people only base it’s value off the FRN, it’s not a real currency.” is uttered even though it is intended to imply that FRN are a real currency and they denominate a real degree of value it actually makes it a valid/true statement: Bitcoin is another commodity, it has scarcity and it stores value, it ascribes to the laws of supply and demand which you ascribed to currency, to and precious metals, and barrels of fish, goats, it is value itself, it only stores value, it has no worth outside a store of value and is not in fact currency at all, it is not a system to measure value any more than a rope is a system to measure length with, or a container is a system to measure volume with, it is only a store of value, it is only rope, it is only a container, not a system or metric to measure with, which is an abstract implying a universally accepted and constant unit to measure with, which money is not, it is bounded by the rules of supply and demand, saying that no matter how much rope or containers you have it won't change the units that are ascribed universally to length or volume.
www.bibocurrency.com