Well, that's something I never heard about, and contradicts even basic high-school chemistry.
Where did you read it?
https://www.atsdr.cdc.gov/phs/phs.asp?id=210&tid=38
Fluoride is one of those ions that diffuse thought and out of our bodies almost like there is no barrier anywhere.
https://www.atsdr.cdc.gov/phs/phs.asp?id=210&tid=38