Calcium fluoride is naturally occurring. Sodium fluoride is a byproduct of industrial processes. They aren't the same thing.
Calcium fluoride is okay in small amounts, but more than small amounts and it is very toxic. Sodium fluoride is questionable in any amount. Definitely not a naturally occurring substance and not something that human beings would ever encounter in the real world. In large or small amounts.
I think it's a common believe that calcium fluoride is naturally occurring and therefore sodium fluoride, which is a byproduct of the production of superphosphates (themselves toxic), is harmless in whatever amounts the experts recommend. Especially since some alternative health practitioners agree. You see, there frequently seem to be two sets of rules. When alternative health practitioners stray too far from mainstream medicine, they are wrong. When they agree, they prove that mainstream medicine is right about the topic in question, because alternative practitioners wouldn't recommend anything dangerous or useless, since they're "alternative". Of course, there is a huge problem of circular logic embedded somewhere in there....