“There are higher concentrations of fluoride in water than we usually put in it that is still healthy to drink” != “Any concentration of fluoride in water is safe”
Any substance becomes toxic if you ingest too much of it. If you exceed by a factor of 20 the amount of plenty of things people usually consume, it isn’t difficult to find things that are dangerous or even lethal. Say, coffee, beer, anti-inflammatories, chocolate, Coke.