Confused!!

Confused!! So i have been reading one self therapy book for stutterers In the beginning, it says we start with the assumptions that there is no physical defect or impairment of speech mechanism that you have which will you stop you from becoming fluent. And the author concludes the book by basically contradicting himself by saying That stuttering is essentially a neuromuscular disorder whose core consists of tiny lags and disruptions in the timing of the complicated movements required for speech. Im confused what to believe and what not to ?