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.