Nonparametric Test: The Genetics and IVF Institute conducted a clinical trial of its methods for gender

Nonparametric Test: The Genetics and IVF Institute conducted a clinical trial of its methods for gender selection. When this textbook was written (2011), 172 of 211 babies born to parents using the YSORT method were boys. If the sign test used, why is it considered to be a "nonparametric" test of a "distribution-free" test?