In practice, because BAEC and QR flight systems don't communicate and the limited staff on both ends have better things to do with their lives than manually check that a passenger flew flight x if they forgot to enter their FFP. The boarding pass will confirm that the passenger was checked in for the flight (though not strictly speaking that he/she flew) and that no other FFP was entered and in practice that would typically be enough for BAEC to verify that the claim is bona fide.