I’m confused on why does 2FA on this casino requires email while it usually done using authenticator app like google authenticator without the need of email verification.
It doesn't, the email is just a confirmation that the payment has been sent.
It works just the way you describe it "using authenticator app like google authenticator without the need of email verification."
If this is correct then why does user needs to wait for the email notification for 2FA? It still fall on the email notification since it relies heavily on email right.