Do you understand that statement?
If not, consider this one: "United States Federal Reserve Notes are fiat currency."
Understand it now?
Why is one fiat and not the other?
What is the NATURE of each currency? (you do realize, they are not the same thing right?)
Now, what is the NATURE of Bitcoin?
There is your answer. It IS a fiat currency.
It is by "decree." It does not represent a commodity exchange on the open market. It is entirely a fictional construct.