Wirraway,
I think you'll find that the certification referred to is certification by the FAA for Royal Tongan to operate as a foreign air carrier therefore precluding them from commencing services to Honolulu. The aircraft in question is maintained by Royal Brunei in accordance with the requirements of JAR145 and is therefore fully airworthy and continues to operate Royal Tongan's other routes. Responsibility for certification as a foreign air carrier must rest with Royal Tongan and I suspect that they were hoping for approval in time to start services but it did not come through in time.