Really??? Show me where Brown has claimed the plot of his novel ACTUALLY happened. I'll wait.
Of course, he doesn't claim that the plot (i.e., about Langdon, etc.) actually happened. However, he does claim that the underlying "history" is genuine (i.e., that there's been a massive cover-up of Jesus' and Mary Magdalene's relationship; traditional Christianity has basically been a hoax; and secret societies like The Priory of Sion have been guarding the truth and planting clues for centuries).