Stable coins are more than those pegged with fiat.
Aside USDT, USDC, DAI and others fiat begged stable coins, stable coins can be pegged with any other thing. Example is wBTC that is pegged with bitcoin price. Another example is PAXG and XAUt that is pegged with gold and so on. They are stable coins because they have the same price or almost the same price with the coins they are pegged with. As the asset go up, they go up and as the asset come down, they also come down. I mean in price.
From my understanding, what make them stablecoin is because of their lack of volatility irrespective of the forces of demand and supply. Even if the price will change, it will just be very small that it will approximately be the same thing. Therefore, the moment you remove volatility from any cryptocurrency, it becomes a stablecoin. It may be tied to anything and it doesn't matter, it just have to be stable and not volatile. Aside USDT, USDC, DAI and others fiat begged stable coins, stable coins can be pegged with any other thing. Example is wBTC that is pegged with bitcoin price. Another example is PAXG and XAUt that is pegged with gold and so on. They are stable coins because they have the same price or almost the same price with the coins they are pegged with. As the asset go up, they go up and as the asset come down, they also come down. I mean in price.
Try viewing it as USDT/other fiat and you will see. They are both unstable and volatile.