You know they'd have a point if they were talking about Bitcoin and gold, but they're not - because if you don't get understand what underpins the value of fiat, or pretend it's the same - then you have an agenda to push.
Fiat has value because it's legally recognized by governments that issue it: it extinguishes tax and debt obligations in those jurisdictions. This makes it markedly different to any other of the commodity items listed: my government will only ever tax me fiat currency, which is sufficient to extinguish those taxes and fulfill debts. I can't be ordered to pay them in gold, or bitcoin.
https://www.lynalden.com/bitcoin-ponzi-scheme/
"Bitcoin doesn’t really meet this broader definition of a Ponzi scheme any more than the gold market, the global fiat banking system, or less liquid markets like fine art, fine wine, collectable cars, or beachfront property. In other words, if your definition of something is so broad that it includes every non-cashflow store of value, you need a better definition."