It pains me to see the word "fiat" used repeatedly this way. Words have precise meanings. The word "fiat" means "by government decree". Fiat money is money that is made so by government decree. It can be paper, gold, bitcoins, notched sticks, or giant stones. If the government of Finland declares Bitcoin to be their national currency, bitcoins would technically be a fiat currency in Finland. "Fiat" does not mean representative money, such as notes, checks, or electronic credits. A private bank could hypothetically issue a note without a government decree, and it would still be "worthless paper", but it would not be "fiat".
I consider 'fiat money' to be money that is *created* by government decree, not merely used by order of government decree. And thus if Finland declared Bitcoin to be their national currency, it would not be fiat money by my definition. Likewise if the US government declared gold and silver coins to be legal tender they would not be fiat money.