مَا زَيْدٌ إِلَّا كَاتِبٌ
Zayd is only a writer.
Ifrād addresses an assumption that two qualities or two bearers coexist. The assumption must be intelligible: writing and poetry can belong to the same person, so denying their joint possession makes sense.
Where the attributes genuinely cannot coexist, a listener’s imagined conjunction has a different logical status. The classification should respect the possible joint occurrence of the alternatives, not merely match a phrase mechanically.