This is his primary problem. When he presents his "facts" he is often not presenting obscure but well established facts, but wide-eyed conspiracy theories that have been soundly refuted and are not taken seriously by legitimate historians or scientists.
If he bothered to do even a little research, and look for original historical sources, such as contemporary accounts or archaeological evidence to confirm what he has read in his fringe conspiracy books (in other words if he bothered reading well-researched and vetted books from credentialed historians), most of his "facts" would evaporate and he would lose his plot devices.
I don't really care that he uses this stuff to create a narrative. It's fiction, after all. But I have a real problem when he presents this crap as fact to an audience that lacks the discernment to know better. He is, in my opinion, intentionally misleading people, and making them more ignorant, to line his own pockets.
Yes. The book had an introductory page headlined "FACT:" that said "all descriptions of secret societies in this book are accurate", or words to that effect.
I think most pop fiction books these days are written more as a script than a book, optimized more for being picked up by a TV a movie studio than for the reading experience. For that you need a good story, especially a conspiratorial one with a twist ending, but not great writing since the screenwriters will rewrite it again anyway. Melville, Hemingway, and Eco they are not.
The problem is that he chooses his "facts" for more interesting plots. He gaves interesting theories, facts and detailed descriptions, but you don't really know what is accepted knowledge or not. It is like getting your news from Fox, you spent so much time filtering it that it isn't worth.
BTW, I've just read Da Vinci Code for the first time after it was one of the few available books in a island after drowning my Kindle. I was impressed how a so successful book had such a poor narrative. His characters building were primary, and did some narrative tricks to deceive the reader. The conspiracy theory thing was the only interesting thing. No more Dan Brown for me. Better to stay with Humberto Eco.