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.