It is misleading to treat khass as meaning “a term that can refer to only one existing person.” Zayd may designate a definite person, whereas rajul names a type and insan a genus in the book’s examples.
A determinate lexical meaning can apply to more than one real individual. That fact alone does not convert it into a general expression in the methodological sense. Determinacy of meaning differs from the inclusion of every member in an asserted ruling.