My guess is that it's being send to the email the user registered with. And requiring them to provide a separate email from the one they registered with completely defeats the purpose of obfuscating the email in the first place, so I think it'd be unreasonable to ask a developer to implement a whole second place to put an email just to work around Apple denying access to that information in the first place.

At the very least Sign in with Apple needs to support a request for the user to enter the real email they want to use to be contacted by the app after-the-fact for cases where someone obfuscates their email but then wants access to functionality that explicitly requires their real email.