In “There is no writer in the town except Ali,” the town is the scope, being a writer is the attribute, and Ali is the person to whom that attribute is exclusively assigned. Even a real restriction must be read within its stated field.
Grammatically, “lā” negates the entire class; “kātiba” is its noun; “fī al-madīna” indicates the location; and the exception introduced by “illā” supplies the remaining affirmed person.
This does not deprive Ali of his other properties. It excludes others only with respect to writing in the stated scope.