It seems to me that many bitcoin proponents use the word "fiat" to describe central bank issued fiat currency, but don't use the word to describe Bitcoin.
In my opinion, Bitcoin is the ultimate fiat currency, since it has no inherent value (you can can't use it for anything other than paying someone else bitcoin). It is a currency issued by "decree" through a protocol enforcing a consensus of users and miners.
If bitcoin is not fiat, can you really call it a commodity currency? If the argument is that is a commodity in the sense that it valuable a currency, one could also say that USD has commodity value in that its only guaranteed value is that it can be used to pay taxes and keep an American taxpayer out of jail for failure to do so.
The value of any money or commodity is affected by offer/demand. The use of a commodity is just a parameter in offer/demand among many others.
Both USD and BTC are backed by the economy around it (=people using it to exchange things).
The supply of USD is strongly affected by a group of people (government) but
not in Bitcoin. The number of miners doesn't affect the emission curve, no miner can change the max number of units.
Miners do not create Bitcoin, they work for the network and get a reward.