This is a valid point, what you are really arguing here though is that the software development group of bitcoin are somehow a recognised "authority" ... if you can classify them as an authority, then yes you are right, it is by their decree, or rules, that bitcoin is fiat.
Now how do you go about arguing that the software developer group of bitcoin are a legitimate authority?
Definition of authority, as used in the context of fiat, not quite a match but maybe ...
The software development group isn't an authority in the manner in which you think. They are only an authority in the sense that people have a certain amount of trust in the software they produce. Also, there is no one software development group...there is different bitcoin related software and different bitcoin clients that completely unaffiliated people are creating. They do not possess any unique position to influence the operation of bitcoins than anyone else. The software development group does not have the ability to unilaterally change the bitcoin protocols without convincing the network of people using bitcoins of the merits of such a change.
It is in fact mathematics and physics that govern the bitcoin system. You only have to trust that your computer runs the code correctly, that the mathematics are sound and that there aren't major flaws in the code. And, you don't necessarily have to trust those things...to the extent you are knowledgeable in those subjects, you can verify it.
And, here are a few definitions of fiat money in case you are still unconvinced:
http://en.wikipedia.org/wiki/Fiat_money ("Fiat money is money that has value only because of government regulation or law.")
http://www.investopedia.com/terms/f/fiatmoney.asp ("Currency that a government has declared to be legal tender")
http://dictionary.reference.com/browse/fiat+money ("paper currency made legal tender by a fiat of the government")
http://www.answers.com/topic/fiat-money ("Legal tender, especially paper currency, authorized by a government")
http://www.thefreedictionary.com/fiat+money ("Legal tender, especially paper currency, authorized by a government")
I think it would be a shame if you didn't at least devote a paragraph to the definition of fiat money (and preferably go on to explain how bitcoin is not a fiat currency, but has no commodity backing...but at a minimum, mention that there are many that would vehemently disagree with its characterization as a fiat money).