The CDC lowered fluoride permissible range levels in water in 2015 from the old range of 0.7ml to 1.2ml per liter of water to the new range which is a maximum of 0.7ml per liter. This was to prevent increasing cases of fluorosis (which mercola mentioned many times). So there is truth in what he says.
Fluoride might prevent tooth decay by hindering bacteria activity, but by washing your teeth only, and not SWALLOWING IT !
Swallowing fluoride might affect your gut flora badly (which is now related to your brain health), and accumulates in your bones making them weaker.