Suppose the listener believes Khalid and Amr wrote the article together. The speaker replies:
مَا كَتَبَ الْمَقَالَ إِلَّا خَالِدٌ.
Only Khalid wrote the article.
Although the wording is unchanged, its target is now a belief in joint participation: this supports qaṣr al-ifrād. If the listener merely cannot decide which of two agents acted, the force is instead that of qaṣr al-taʿyīn, settling the uncertainty.
Listener's prior beliefRhetorical reading
Amr alone wrote itReversal (qalb)
Both wrote itExcluding partnership (ifrād)
Which of the two?Determination (taʿyīn)