You're asking me to read their minds, but there is a tendency among some security forces to hype their exploits and successes. Once they had excitedly claimed that the terrorists had planned a "chemical bomb" that "could have killed 20,000", it was hard to retract and the press loved it and ran with it.
Since you asked me to speculate, my guess would be that the claim originated from a reporter talking to a low-level, on-the-scene source, within Jordanian security and the higher-ups were too embarrassed to change the story later, as the facts became clear.
--Boot Hill