They probably have the multisig as well but these hackers just understand these things and they're looking for which one has got the lesser security.
I think mere presence of multisig does not guarantee that entire infrastructure will be secure. It is better to keep signing keys in separate location. However, if any attacker somehow manage to gain control over withdrawal automation or privileged credential then problem still persist. They could then drain wallet by leveraging transaction approval workflow.
In my opinion, casino should not keep more liquidity than is necessary in its hot wallet.
Also it would have been better to have independent approval process for large withdrawal.
And if any abnormal outflow is detected then transfer can be halted using automated circuit breaker.
However, not all risk are technical. Credential theft, compromised server, excessive privilege and weak monitoring all of these create large attack surface.