And furthermore, the vast majority of people in the USA are not having too little salt.

The vast majority of them have way way too much salt in their diet, therefore it is perfectly honest to say that high sodium content in food in the USA is "bad for you", because we live in a context where too much salt is the problem, and not too little.

Arsenic? Cyanide? Mercury?

They're being pedantic (probably to highlight the complete lack of useful information provided by the post they responded to.) "Too little" means "not enough" and "not enough" is not enough by definition. There's no such thing as "not enough" of those things because none is enough.