Dictionaries are not written by goldbugs, and while dictionary definitions vary from each other and from prevalent usage over time, when dozens of sources over 80 years all emphasize the same decree aspect, and you are in fact complaining about people widely using it that same way, then both linguistic prescription and linguistic description agree: that is in fact what it means to competent English speakers.
I fully agree that 'fiat money' also has a strong denotation of 'unbacked', because at its origin and through its history, that always been one of its qualities. But that is in addition to, and often reliant upon, the government fiat – not exclusive of the fiat.
Really, the best clue to the term's origin and traditional use was the oldest I quoted, from a non-American source 80 years ago. It describes 'fiat money' in almost exactly the dimensions that you accuse goldbugs of making-up in recent years, which suggests that instead you are making things up.
And yes, Bitcoin isn't backed by a commodity of other value, and yes, it shares that property with fiat money. If you'd simply said that – it is similar to fiat money in this particular way – you'd be on firm ground. The deception you've attempted is the idea that lack-of-collateral-backing is the defining quality of 'fiat money'.
If people using 'fiat money' in its longstanding sense sound ridiculous to you, it's your stubbornly-eccentric definition that should change.
Cite dictionaries all you like...you'll also notice that all but one of those definitions center on paper money or coins, which is an equally irrelevant detail, unless you mean to imply that US dollars are not actually "fiat money" if they aren't ever printed on little bits of paper.
In other words, stop being a pedant: Bitcoin isn't backed by a commodity of any intrinsic value. The value is decreed by the people who accept it for trade, and Bitcoin shares this property with all other "fiat money". I don't need to convince you of this fact, I'm just here to remind you that you all sound completely ridiculous when you use words incorrectly.