Is Spielberg's 'Munich' Based on a Fabrication?

I found this interesting posting on The Internet Haganah website, following links from the Jawa Report story below. It seems they have found an item published in the Guardian that throws the credibility of Spielberg's 'Munich' into question:

Israeli Yuval Aviv, who teamed up with Canadian George Jonas to write Vengeance, the bestseller on which Spielberg's "Munich" is based, never served in the Mossad or any Israeli intelligence organization. His nearest approximation to spy work was as a lowly gate guard for the airline El Al in New York in the early '70s. The tale he had woven was apparently nothing more than a Walter Mitty fabrication.