مَا زَيْدٌ إِلَّا كَاتِبٌ
Zayd is only a writer.
Suppose the listener thinks Zayd is both a writer and a poet. The answer “Zayd is only a writer” affirms writing and rejects the supposed joint attribution of poetry. This is qasr al-ifrād.
The issue is not the number of writers in the world, nor an unrestricted denial of all Zayd’s properties. It removes one of two attributes the listener imagined to be combined in this particular person.