If some people thought that a fictional book sold as such contained true things, then they are gullible fools likely to fall for anything. I wouldn’t read too much into that.

You're welcome to think so, but that doesn't change the fact that Dan Brown was unambiguously purporting to communicate factual reality.

More so that Gladwell, in fact - I don't think any of Gladwell's books ever had a "the following parts of this book are historical fact" preface! (though a few of them could arguably have used one...)