Those casinos don't invest in crypto coins, so ideally they convert crypto coins to stable coins or fiats regularly. The team members of the casinos must also know well about crypto, probably they are the people who have a lot of experience in crypto industry. They won't be brave to utilize crypto coins in their casinos if they don't know well crypto coins. So, I'm sure they won't experience a decrease in their fund reserve during the bearish period.
Adding to that, these crypto-casinos, especially the big ones know already how to play with the market regardless of the market trend. Specifically referring to their respective bankroll, I'm sure they are always prepared to accommodate any big withdrawals of their users. Besides, since it's gambling, it just makes sense to think that their revenues are just growing every day even at some point, there will be lots of big withdrawals that will happen.
This is not the first time that crypto has experienced a big crash and most gambling sites are already used for that.
Bitcoin's bottom price today is even considered just a dream price before.