I've seen instant exchangers and payment processors recycle addresses, and I've paid to addresses that had been used before.
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Screenshots can be faked, signed messages can't. That doesn't prove I have anything to do with the first payment.
That address that is supposedly Rollbit's hot wallet contains over $1 million worth of ETH tokens. I am quite sure that there is a limited number of people that have access to its private keys to sign messages and prove ownership of it. I doubt the person controlling Rollbit's Bitcointalk account is on that list.
There is another possible solution. If OP has used an exchange or a service that recycles addresses and everyone deposits into the same account accompanied with a note to differentiate different people, he should tell us what service it is, we could check it out and create dummy accounts. And if it's true, we should get the same deposit address as OP got. That would then show that the evidence can't prove Rollbit can claim OP has had previous accounts on the casino just based on the same withdrawal address.
Even if that address changes every week or so, two people creating dummy accounts in quick succession should still get the same deposit address.