In order for Bitcoin to be considered a currency it must satisfy three conditions: 1. be a store of value, 2. a medium of exchange and 3. a unit of account.
What separates fiat currencies from BTC is the fact that they satisfy the most critical condition: being a unit of account. The US Dollar is considered a real currency because it satisfies that critical condition. 1 USD is just that: 1 USD. It is backed by nothing other than the full faith and credit of the United States Government. It is not a derivative.
BTC, on the other hand, is a derivative, and is not a unit of account, as it is backed by the nominal value of the USD, which, in this example, is the independent variable.
In order for something to be considered a unit of account, it must have an independent value metric. If it does not have an independent value metric, then its value, by definition, is derived from some other thing. When the value of an asset is derived from the value of an underlying, that asset is defined as a derivative.
