Before editing a response, isolate the point in dispute. A listener may deny that a letter arrived at all; alternatively, the listener may falsely attribute its authorship to Khalid rather than Amr. The two cases do not call for identical reinforcement.
قَدْ وَصَلَ الْكِتَابُ.
The letter has indeed arrived.
مَا كَتَبَ الْكِتَابَ إِلَّا عَمْرٌو.
No one but Amr wrote the letter.
The first strengthens event occurrence; the second restricts the agent by excluding alternatives. Adding inna to an ordinary assertion would not itself negate the mistaken attribution.