use NON AT&T email for Internet Account user ID???

Inside MyAT&T profile for AT&T Internet account, by "Change the UserID," there's a "tip" or suggestion that says about choosing an address (paraphrasing), "It's really better to use an address that's NOT AT&T email."

Why would that be? Or is it a serious typo?

If I remember, years ago any AT&T account login, that used email for a user name, HAD to be an AT&T address.

Thanks.