The reason I have doubted James's sincerity at times is because he sometimes presents information that is so far removed from conventional wisdom that one cannot help but be skeptical, and furthermore does so as if he were telling you what color the sky is. It seems that if he were indeed genuine and rational he would at least acknowledge the unusual nature of his claims so as to help deter claims of intentional trolling or outright insanity. (I realize that one could be simultaneously irrational and genuine, of course.)
But ultimately Fluffy is correct, it is presumptuous to make claims about another person's sincerity. It's just that it's a response that is difficult to suppress sometimes. And I mean no offense to James, he seems like a nice guy to me.