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there is only a small chance that it could happen at an established and still operating casino. but for casinos that might go bankrupt or be indicated as a scam, it is very possible to happen.
I have several times received incoming emails from casino platforms that I don't know. and as far as I remember, I only used that email to register for the casino, not for other activities. they offer deposit and spin bonuses on certain games, which we usually see in casino promotions to new users.
I don't mean to accuse all casinos of doing this, but I often create new casino accounts just to try surfing. maybe one or two do.
Leaks of customer data including emails, cellphone numbers, home addresses, dates of birth are not only due to the actions of irresponsible casinos (if you have ever registered an account at a random casino). Data leaks can also occur in various ways apart from the possibility of one or two casinos selling their customers' KYC data. Casino hacking also allows data leaks and hackers to sell them on the darkmarket, it is common and has been frequent so far.
Of course we're not accusing one or two casinos of being rogue, but this has probably happened before. In fact, I quite often get messages from unknown numbers on WhatsApp about casino offers or other things, it happens and I'm never sure where they got my phone number. In the email, of course there were many offers from various sites (not only casinos), but I never got involved in them, let alone created an account.