Are you being intentionally obtuse? You asked why salt, he answered with literally the most important reason in public health: people actually use it. It is a known delivery vector that works, period.
And, yes, no shit. How to get other people to do something is a fundamental question in life and it just so happens that in life, public health is about getting people to do stuff that's good for their and society's health.
And, yes, no shit. How to get other people to do something is a fundamental question in life and it just so happens that in life, public health is about getting people to do stuff that's good for their and society's health.