This is exactly the other way around. You have no clue how far some people can go, and this has been experimented with in the past many times. Even for just 1 USDT, people have been trying to create hundreds of accounts, even knowing that they can't withdraw it.
The one who has no clue is you and the owner/developer of Blackjack.fun. It is nothing new that you always blame others for your incompetence.It’s too harsh but I will not defend them this time for their current promotion that really sucks.
I understand why they come up to this kind of very strict safety measures because they become a victim of abused of multiple account that manage to withdraw funds out from their casino. But this is not a reason to make the requirements too overkill just to avoid potential abuse since other casino can do this kind of promotions without any issue.
Why not just improve your security like what @Jackpotrace suggesting.