This seems to be a different case
The sender was authentic because emails came from a legit source which had been hacked (at least, as far as domain names are concerned). In this manner, it is not a typical phishing attack (if at all) as you would find no difference in domains. That likely explains why people fell victim to this attack as they saw that the source was authentic but they didn't expect it to be compromised (hacked). Really, if you trust the casino with your money, you would trust emails received from them unless they ask you something which they explicitly made clear in advance that they would never ask under any circumstances
How can you know the sender is the real one? Because I have had similar situation before and I got prefectly the same email that asking me to send some money because he said that I was winning like 1 btc from lottery from X site. And the second time my email was hacked and the second site ask me if I have asked them to retrieve a new password? Because it has exactly the same email and the same name asking for the new password. So I think everything is possible with the hacking. So like you said to trust everything on what they said on email(perfectly copied email from the site you are playing), this wont do good things for you