مَا أَنَا ضَرَبْتُ إِلَّا زَيْدًا
I struck no one except Zayd.
«Illā Zaydan» explicitly restricts the object of striking. If we additionally treat fronted «anā» as conclusive proof that someone else performed the action, we risk conflating two distinct directions of restriction.
The point is to analyze the force of negation and exception independently of any contextual force of fronting. Restricting the object is not the same as assigning or denying the action to a competing agent.