Are coin send with wrong network actually lost?
The casino should have access to the deposited coins. Even with different networks, the deposit address will remain the same. However, they may require certain considerations to continue assisting their customers in crediting the deposits made.
I have never experienced such a situation. It is important to double-check all transactions that will be made.
Fact is that for the story cited by op, the sender isn’t just at fault but have repeated the mistake hence even if the casino decides not to credit him or her it’s very understandable but fir the casino to be biased enough to say they don’t have access to the third party account it’s also on the side of the casino that they are purely responsible for the coin but don’t want to release it hence covering up with some lies.