Value of anything only exists in the minds of humans. No value is inherent to anything. You’re absolutely correct that gold isn’t fiat. But gold could absolutely be used as currency. Its in circulation and used as a medium of exchange. You’re also right that crypto is a currency. But its not a fiat currency. Crypto and fiat are both mediums of exchange not backed by a commodity. The key distinction is that fiat also relies on trust from the issuing institution.