Whether or not OP has multiple accounts I wonder why they come to this conclusion now AFTER the player deposited and LOST half a million $ ? Now that he won a fraction back they suddenly find out that he has multiple accounts? Sounds a bit odd to me to say the least.
Sure it's possible but why allow him to play and LOSE in the first place.
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I can explain this (even though my knowledge of this dates back to 2017, but I don't think anything has changed here):
Non-KYC casinos generally do not carry out checks on deposits. As a casino user, you can therefore also create umpteen accounts, deposit 10 BTC each and play with them. The violation of the ToS (multiaccounting, ...) will not be noticed here and if it is, it will probably even be tolerated, as it usually makes the casinos money.
However, if a user then pays out money, the so-called “Extended User Checks” will kick in. These not only include checks against the ToS (multiaccounting, etc.) but also detailed analyses of gaming behavior in order to be able to prove abuse, for example. Only when all checks are OK and the necessary regulatory data has been obtained (KYC, proof of residence, etc.) is the payout released. Incidentally, this can also happen with 10$, but the higher the amount, the more secure the checks are.
This means that players can play (and lose money) for a long time. From the casino's point of view, this approach is understandable, of course, as it allows them to make money and not scare off potential customers in advance. Whether the whole thing is ethically correct is of course another question.