People who still smoke nowadays aren't even deterred by the very graphic images of internal organs affected by smoking-related diseases that are printed on cigarette packs in many countries (next to very clear wording that smoking will harm you and people around you). So I doubt that this would make much of a difference in that regard.

Probably not. They already know the outcomes and don't seem to care.

However 71% seems like a very low number... Does that mean that in 7 out of 10 cases the system guesses correctly if the patient is smoker or not? That's not really impressive IMHO. What am I missing?

what is impressive is that humans cannot do it

They can get to 50% with a coin.

I seriously doubt that, just reflecting on how smokers so often remain unswayed¹ even by far more brutal prognostications about the habit's effect on their longevity.

¹well, unswayed about actually quitting now

certainly interesting info for health insurance firms