The source describes real restriction of a subject to one attribute in these words:
«وَهُوَ لَا يَكَادُ يُوجَدُ»
That is, it is scarcely found in ordinary usage.
A real person cannot normally be said to possess only a single property while lacking all other properties. Zayd may be a writer, but he is also human, alive, capable of speech, and someone’s son. An unrestricted denial of every other quality would be extraordinary.
The sentence “mā Zaydun illā kātib” must therefore be understood against its actual dispute. It most often selects one attribute among the alternatives under discussion, rather than denying every other possible attribute.