Why is mā anā qultu hādhā wa-lā ghayrī awkward under that intended restriction?

Fronting the Musnad Ilayh: Prominence, Restriction and Reinforcement

• Because it first contrasts another agent, then denies every alternative agent
• Because both clauses assert the same speaker
• Because wa-lā ghayrī explicitly affirms the first agent
• Because mā eliminates the fronting of anā