Like some said if you are winning big it is a custom that the casino will ask for your KYC just in case if you are playing from a country that they think is banned they can get away without paying them citing TOS and they do not mind keeping the amount if they loose, so that is basically with the KYC and we are playing with bitcoin and the idea of they are going to transfer huge amount does not mean that it is mandatory to verify your details under any law that i know off as we do not have a law in many countries regarding cryptocurrency.

@OP congrats on your huge win and hopefully they will credit your amount soon rather than keeping you long.