In fact, Bitcoin is the only one that takes a long time to confirm a block. Apart from that, other coins complete transactions very quickly, due to which when someone deposits a crypto on a casino site, it is credited to the balance immediately because the transaction is also completed quickly and there is never a chance to cancel the transaction. But in the case of Bitcoin, as far as I have seen, it takes a little longer than other chains to make a deposit. Still, they credit the balance before the block is fully confirmed. I don't know how they verify a valid transaction, but I have not seen any casino site spreading news that they face any problems for pre-credit or they have to face any kind of loss.