Perhaps not, but you most definitely have a problem with admitting when your reach has overcome your grasp. Is it that costly for you to admit making an assumption that was invalid?
It's a diet book, not an exercise book. But it seems obvious you're not going to concede the point with anything approaching polite correction.
He wasn't some regular schmuck just doing a diet book, he was a Doctor, for crying out loud, that changes the equation a little bit, IMO.
But here you go, I was wrong you're right, it was a diet book, end of story.