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?
Define the sender here
If by that you mean an individual from the casino staff and not the scammers, then you can't know that. But you can't prove otherwise either as the sender's address is a legit one, i.e. the one belonging to BitCasino. In other words, this is not what is called address spoofing. As the Internet suggests, "spoofing is the act of disguising a communication from an unknown source as being from a known, trusted source". But these emails were not from an unknown source but from BitCasino itself (technically, from BitCasino's email service provider which had been hacked)