I believe they can not sign a message with this amount of bitcoin because they dont have it or they do not know how to sign a message in the first place.
Just coming up with another excuse toward their customer to delay paying him once again.
This must be a joke LOL
You would be surprised. I asked another casino to do it several years ago and they did not know what I was talking about.
They are a casino and only interested in receiving funds and nothing else.