Bitcoin is not a currency because it is not a unit. It has to be definable it has to be equal to something. An inch is not real but it is definable.
This whole conceptual construct is faulty, but just to humor this "unit" nonsense that has nothing to do with anything but looking up something in the dictionary then making stuff up --
A Bitcoin is a unit of what you can transact that the network will accept as legitimate and incorporate into the blockchain.