You are correct about that high conc of fluoride is harmful. Fluoride in toothpaste is in the parts per billion area, very dilute. But you have to consider that you re exposed to fluor everyday in your food, in your drinking water etc. Fluor is a common element in the solar system. So why dont we get poisoned you ask? Some elements (or all?) is acting like poison in too high concentrations and is also killing you if you get too little of it. Your body needs a constant supply of iron for your blood, but too much iron might kill you. So goes for the fluoride, but in ultratrace amounts. We all need up to 12 elements in correct dose for an healthy life.
Read about the Bertrand rule.