Many many books are based on real events, and introduce fictional characters and twists on truth, or downright falsehoods. And if he believes some aspects of his book are true (as some believe the Illuminati and the Elders of Zion control the world), he'll say so.
And since Dan Brown did say so, he deserves to be praised or criticized based on those statements as well as the reasoning and evidence he provides for them, just as D.W. Griffith's Birth of A Nation deserved to be so criticized.