It's a bell curve. Fluoridation of drinking water can be both adding or removing fluoride from the water supply. They looked at the average amount of water people drink and the impact on one particular parameter (tooth decay) they cared about and then they just picked a target concentration they try to hit. You're on a bell curve somewhere for how much tap water you drink and for how sensitive to fluoride you are. And if you're an outlier, well tough shit.
It's a cold, impassionate numbers game; they think it will help more people than it will hurt people, and they know and deliberately accept that what they are doing will hurt some people.
What he's saying is both obvious and true. The people who are having 8 kids are doing so because 4 survive childhood illnesses like measles and the kids are their retirement plan.