Which method was he referring to? What studies did he cite?
Each method is distinct. Doctors have studied each method. You should research Sympto-Thermal, Marquette, Creighton, and Billings individually. These clinical studies do show very high effectiveness.
Websites citing supposed effectiveness rates of various methods of contraception typically lump NFP in with “no method”, “rhythm method”, “fertility awareness method” and sometimes with “withdrawal” as well.
I haven’t ever seen any curated effectiveness rate statistics that list the natural family planning methods by method and separate them out from these other completely unreliable unscientific methods. I mean “no method” really brings those stats down, don’t you think?