I could quibble with that as a definition of "fiat," but in the end whether Bitcoin is or isn't "fiat" is not a substantive point. The fact that central authorities can change — or even influence — the size of the supply of money is a substantive point.

As it happens, taking that power from those authorities is the point of a proposed change to the centralized monetary paradigm that I have developed. The supply of money (as currency) would become self-regulating, making the economy self-regulating--while eliminating unemployment, poverty, and taxation/public debt and increasing sustainability (at no cost to anyone, without having to redistribute anything, without imposing any limit on income/wealth, and without requiring people to change their behavior in any way). That change to the existing system would have no necessary effect on the existence of Bitcoin, etc.

If curious: "Easy as 1-2-3-4" (here in Medium).