Gold is a real thing that has value though. Bitcoin literally stops existing if you stop believing in it.
That is the case for countless things. Is a corporation a real thing or a belief? Where is the physical entity Facebook or the physical entity Apple? Is it the people working for them? Their logo? Their contracts? It's all that and a collective belief in an abstract entity. That's true for fiat or states too, even if that belief can be enforced through e.g. army, that doesn't change the fact that's is a collective belief in something that has no material reality.
If your state demands your taxes be paid in FIAT, no matter what you wan't, you'll be paying in FIAT (or fined, or in jail, or depending on the circunstances maybe dead).
When the taxman comes knocking on their door, do you think this person will escape by saying "sorry, I don't have fiat available to pay for the taxes, come back next year!"? No, the taxman will respond by either seizing their assets or forcing the person to liquidate into fiat.
What makes Bitcoin (or crypto) different in that regard?
bitcoin acts more like a commodity than fiat. Bitcoin is backed by the laws of mathematics, rather than laws of nature (which is the case for gold).
Your claim that bitcoin is not fiat because it's backed by the laws of mathematics makes as much sense as claiming that the dollar is not fiat because it's backed by the printing press. This is not what 'backed' means in the context of money.