USDT, USDC or other stable coins are still part of cryptocurrencies. They are pegged to fiat prices but people still see them as cryptocurrencies.
You made a valide point right there, stablecoin are still cryptocurrency, and there are performing the functions as the global payment on those casinos, less not forget the purpose for cryptocurrency adoption in gambling is to aid gambling platforms payment scalability, and to avoid the regional restrictions that comes with using fiat for transactions.
You both highlighted the most interesting aspect and they were well expressed from your opinions, the use of stablecoin is not the same as the use of fiat, the only similarities they may have is that they both have the fiat USD value for their equivalencies, while fiat remains fiat and stablecoin a cryptocurrency, even as we all know its not volatile, we can choose form any crypto coin to gamble and there is nothing appearing to change from it.