How Can Dan's Brown Da Vinci Code Claim About The Existence Of The Priory Of Zion Be Debunked?

In his book the Da Vinci Code, Dan Brown distinctly claims at the beginning of the book that The Priory of Sion is a real organization, discovered in 1975, when Paris's Bibliothèque Nationale discovered parchments known as Les Dossiers Secrets, identifying numerous members of the organisation. In the story, The Priory of Sion is the guardian of the Holy Grail in the narrative. However, these claims are not fact at all.  

In fact, four Frenchmen founded the Priory of Sion in 1956, one of which admitted on the BBC in 1996 that it was invented by the group for a bit of fun. Another founder admitted in court in 1993 that the Priory of Zion documents were fabrications. But again Brown tells us as background that the Dossiers Secrets have been authenticated by many specialists. This signals one of the many times when the reader becomes confused between fact and fiction.