I don't know if that's it or not. Maybe that's partially it, but at the same time...maybe not all of it. I'm kind of an insider in this world. I work in biomedical research, and there are plenty of other researchers and even MDs I know who smoke cigarettes. Well 100% of them believe that smoking increases risk of cancer and heart disease. They still do it, though, for various reasons all of which fall under the umbrella of even smart people are still human with vices. Also, in my field, the notion that sugar is extremely bad for you is not extremely taboo and hasn't been for a while (in spite of the food industry backlash against Lustig). However, we still have pastries and coffee every other week at lab meetings.
So really no it wouldn't mean that they'd have to give those things up. Just because you know something is bad for you doesn't mean you HAVE to give it up. People do of all professions, from doctors to day laborers, do things that they know are bad for them, and they do them all the time.

I think it's more intellectual than personal. The idea that grains aren't good for you and fat isn't bad for you goes against what you've learned, and possibly worked on for years, so the idea that everything you know is wrong is a hard thing to accept.