A fiat currency is issued by a government without being backed by a physical commodity. El Salvador's adoption of Bitcoin does not make it a fiat currency.
I think this is in reference to a US law that states that a "currency" adopted by a foreign country is not classified as an asset but as a currency. This has tax implications but I don't know the details.
It's a bit of a weird situation. Normally when a currency is legal tender and not fiat it's backed by a physical resource, like gold or silver.
But here it's only backed by software and people agreeing that it has value. Kind of puts bitcoin in its own unique category.