Are we changing the definition of fiat currency now? No. Fiat currency is a currency not backed by any physical commodity.

Fiat currency is defined as:

• Fiat money is a government-issued currency that is not backed by a commodity such as gold.
• Fiat money gives central banks greater control over the economy because they can control how much money is printed.
• Most modern paper currencies, such as the U.S. dollar, are fiat currencies.
• One danger of fiat money is that governments can print too much of it, resulting in hyperinflation.

None of these points matches cryptocurrency. Bitcoin itself is a type of asset class that doesn't need backing. Try use your brain for once, now imagine you have a gold coin in your hand, a bullet, a grain of salt or whatever, now what makes it's valuable?

Notice that you can't just put your hand out and magically spawn any of those things. Same with crypto, you can't create it out of thin air, it requires physical work or energy, some form of investment from your end. There's the value.

Fiat on the other hand you can print out of thin air, but if you're not a member of the cabal they'll beat you up and steal your currency. "Backed by" is a lie worth as little as "Dude trust me".