Look at the fact that there are some bitcoin casinos did it already. Casinos such as Bitsler and Fortunejack has the system to detect money laundering attempt, the site hold users withdrawal because it was detected as money laundering attempt. Money laundering does not mean that any sites needs to know the real identity of the users but the sites knows the activity. The main idea of money laundering is not about who are the user but about the attempt. If you think that money laundering is only about who are the user doing it then it is a big mistake.
I bet the detection is when people just deposit their funds and then withdraw it without never playing. So if the launderer play nicely then the system could not detect it right? That means without KYC, gambling sites are not actually able to prevent money laundering. They are just assuming, and maybe people will be victim of their weak assumption.
If at the end, the sites asking for identity verification before withdrawal then why don't they just apply it from the start so that bad people won't even use those websites, there will be no money laundering
attempts, lol.