mBTC denomination is only for whales, even uBTC denomination is too expensive for me. I did uBTC bet when the price was $6k-$7k, but now it's just no way!
Therefore, it has no effect as long as users can deposit BTC and play with its fiat value, which is pretty much best practice on crypto casinos.
Anyway, if they really remove BTC as deposit/withdraw method, then it's their loss as we can pick other hundreds of crypto casinos to play.