I have no doubts as to the sincerity of Philip Johnson, an obviously intelligent man.
One doesn't have to be stupid to be deluded. Emotions can prompt some of the smartest people to believe the most absurd things, against all reason and evidence. How else can you explain that a man smart enough to earn a Ph.D. in biology from a top university was deluded into becoming a Moonie?
I know some extraordinarily intelligent Mormons (and I even support one for president) who accept a religion that is almost laughable in its absurdity, though it's not quite as absurd as the cult of "Reverend" Moon.
I suspect the vast majority of the ID and creation "science" crowd, all demonstrably intelligent people, are similarly deluded.
FWIW, Kant said that stupidity was incurable by any amount of education and a PhD would emerge just as stupid as he went in.