I would imagine that it may have been known, "by person or persons unknown", that Primary radar would still be able to track the a/c.
Or perhaps the "person or persons unknown" descended to 5000ft on a westbound heading until they knew that they were clear of military primary radar and then turned south and climbed to cruise altitude. From what I understand, the military radar didn't notice them at the time but the replay showed the flight heading west when they lost contact.