I was speculating on the cause of this effect, not disputing it.

As for whether he believes in these conspiracies ... well, I know for a fact that they're based on real conspiracy theories with actual proponents, although I'm not sure where I heard that he believes them himself. I suppose he might be simply using them as inspiration; I'll try and track down my source on that.

ETA: a moment's quick googling reveals that

Much criticism also centers on Brown's claim found in the preface to The Da Vinci Code that the novel is based on fact in relation to Opus Dei and the Priory of Sion, and that "all descriptions of artwork, architecture, documents and secret rituals in [the] novel are accurate." [Wikipedia]

This may be what I was thinking of.