You need a method of restoring the subscription to a new device purchased by the user who has a subscription. If your app requires that the user have a user account, and through that user account they can restore, then you do not need another means.
>but there is no "official final word"
From Technical Note TN2387:
"Provide a UI for restoring products
If your app sells non-consumable, auto-renewable subscription, or non-renewing subscription products, then you must provide a UI that allows them to be restored. See Differences Between Product Types and Restoring Purchased Products for more information."