Unfortunately, not everyone knows about this. Otherwise, people wouldn't use the secret question to recover their accounts. There is no point in learning more since they have already been blocked from using it. But other people may find it useful. As for signing a message from a Bitcoin address, I think this is one of the easiest methods. But when it comes to commercial companies like a casino, usually, they do not post their Bitcoin address anywhere. So, they cannot prove anything by signing a message from their wallet. Since they are commercial casinos, all they can do is send an email from their official domain email. For example, they can send an email from contact@bspin.io to prove the ownership. The whole point is to prove the ownership of the account.