He means that his gmail account was set up to mail a password reset link to a backup email account, which was in this case his apple mail account. After that, a

He means that his gmail account was set up to mail a password reset link to a backup email account, which was in this case his apple mail account. After that, all his other accounts were reset by clicking on "forgot password" or the equivalent, which sent password resets to his apple or gmail account.