Who cares?
FWIW I use my free iCloud email account with my custom domain since Apple (unlike Google) offers that as a free service. However, I just forward all email to my GMail address and use a custom reply-to in GMail so it _seems_ like I'm using the custom domain in both directions.
Doesn't Apple helpfully offer logging in with their email address constantly? At any rate, they were receiving emails on it and I don't think the user knew how to add an email address. They couldn't find their email client's account settings in iOS, but they struggled with several inboxes in the email client.
I assume that when you start using the device (since that requires Apple account tying) they create a new 'free' email address and provision it onto your devices. From what I later found online, indeed Apple has separate settings for their cloud offerings as compared to when you buy it from a competitor, which afaik is anticompetitive but until all countries get around to sending desist notices... apple does what apple does best