By any reasonable definition, Bitcoin is a fiat currency. Unlike commodities like gold or silver, it has no inherent value whatsoever, and no applications for industrial use or jewellery. Bitcoin is a fiat currency because it only has value while some group of humans collectively agree to pretend that is has value. The same as dollars or yen.
The key difference, though, is that fiat currency is easily debased because whoever controls it can print more, while Bitcoin does not have a single owner and can only change if the majority of miners agree to do so.
I’d be interested to know how many people would actually need to agree a fork that, say, didn’t halve the coinbase.
Also, of those, how many would give priority to the philosophical underpinnings of Bitcoin over making more money in the short/medium term.