I don't think bitcoins are an economy by itself. If you say economy it is something that is produced and consumed by the people.
In bitcoins there is only a medium of exchange, nothing else. There is no economy to speak of here. The only thing bitcoins would
be in an economy is a way to pay for stuff, but it can't produce stuff, nothing goes around.
You're quite right, I guess. Bitcoin, as of the moment, can be simply put as a part of the economy, not the economy itself. It's all about transactions, exchange of money and value of bitcoin. But on the other hand, bitcoin can really make a lot of impact if it is considered legally by the public and government as associate or part of ones country's economy.