I think this is just a regular KYC probably due to his withdrawal pattern after profit or regarding the method on how he get that profit.
That doesn't make any sense.
Casinos cannot request KYC just because of certain withdrawal patterns unless they suspect money laundering or something which doesn't seem to be the case here.
They can check how he acquired the profit by themselves without requesting any form of KYC basically.
They required KYC because they saw something unusual on this account or else this guy can withdraw without KYC since I have Duelbits account that still can manage to instant withdraw without any KYC.
And also casino can ask KYC using whatever reason they want without giving further explanation to the customers. This line is indicated on ToS of every casino. Your idea that casino can't request KYC is non existence in reality.