It looks like you're confusing a review with a scientific study. The review in the Lancet was just that - not a primary study on the toxicity of fluoride. There is a pretty big difference.

Fluoride was only one of 11 chemicals and was barely mentioned. Also mentioned in the same review, manganese. Here's what WebMD says about manganese:

"Manganese is a mineral that is found in several foods including nuts, legumes, seeds, tea, whole grains, and leafy green vegetables. It is considered an essential nutrient, because the body requires it to function properly. People use manganese as medicine.
Manganese is used for prevention and treatment of manganese deficiency, a condition in which the body doesn’t have enough manganese. It is also used for weak bones (osteoporosis), a type of “tired blood” (anemia), and symptoms of premenstrual syndrome (PMS)."

"Untested chemicals should not be presumed to be safe..."

Fluoride has been studied extensively.