I can easily see a small child, at that age where they haven't yet developed a brain-to-mouth filter, blurting out the lesson about smoking he'd learned in school. It doesn't make it okay, but it is understandable. If a parent/caretaker was in earshot, s/he should take the child aside and tell him that yes, smoking is bad for you, but it's not polite to go around telling random adults that. Grown-ups have the right to do things that aren't good for them, such as smoke, and the fact that it's bad for them isn't news.
I wonder if the kid had been subjected to the DARE program (is that still a thing?) or something similar. We had DARE when I was in elementary school (late '80's/early '90's) and they basically told us that drinking a beer or a glass of wine made you an alcoholic, smoking so much as a single cigarette would kill you, and just THINKING about consuming weed or other illegal substances would cause you to become addicted, homeless, criminal and die a horrible lonely death. I, and a lot of other kids, became concerned that our single glass-of-wine-with-dinner-drinking parents were on the verge of becoming abusive, violent alcoholics, because the program failed to draw lines between moderate drinking for grown-ups and alcoholism, as well as to distinguish between, say, beer or cigarettes and crack or heroin. It's ridiculous, scares kids, and leads to stuff like the kid in the letter.