Actually that is not true.  Anyone who has the card number **AND** the card security code can use the balance whether it is loaded in the app or not because the app does not change the cards numbers and does not lock the codes.  If you (or a scammer seller) loads the old codes money onto a NEW CARD at a store or send yourself an e-gift with the original codes this will create a new card with a new set of numbers and zeroes out the old card then what you say is true because the seller no longer has the original codes since they were moved to a new card.

If you buy cards you should always move the balance to a new card with a new number to protect yourself from a scammer as soon as you see fit.  Remember though, doing this will stop a scammer seller from reusing your codes but will not prevent Starbucks from killing off the card if the seller obtained it illegally (and we know no seller here would do that) as the money can be "tracked" from card to card.