Maʿdūlah does not mean that the overall proposition must be affirmative. The book explicitly divides both affirmative and negative propositions into metathetic and non-metathetic forms.
Consider “This person is not a non-scholar”:
Predicate: “non-scholar,” a negated concept.
Connection: “is not,” which denies that predicate of the subject.
Thus a negated term and a negative judgment can occur together. They are not redundant simply because both contain negation; their logical roles are different.