Well, by definition, it cannot be fiat money if it is voluntary. "Fiat" indicates a government mandate, decree or command... in short: force. As is the case with legal tender laws.
As in "fiat lux" = "Let there be light" (by divine will...). It implies that governments have the power to make something just by their almighty say-so. It's almost a flashback to the divine rights of kings.
noun
1. an authoritative decree, sanction, or order: a royal fiat. Synonyms: authorization, directive, ruling, mandate, diktat, ukase.
2.a fixed form of words containing the word fiat, by which a person in authority gives sanction, or authorization.
3.an arbitrary decree or pronouncement, especially by a person or group of persons having absolute authority to enforce it: The king ruled by fiat.
It always amazes me how many people who claim to "know money" have not yet learned the real meaning of fiat: value by decree.
It's fine if someone wants to say Bitcoin is "not backed by anything" (intelligent people can argue this both ways)... but it is inaccurate to say it is "fiat."
when a supposed "expert" dismisses a new idea so carelessly you know they have a vested interest in doing so.