Parsing the Preposed Exception

Al-Mustathnā: Connected and Disconnected Exceptions, Four Case Rules

«مَا جَاءَنِي إِلَّا زَيْدًا أَحَدٌ»
mā: negator.
jāʾa: past verb.
-nī: protective nūn plus object pronoun.
illā: exception particle.
Zaydan: preposed accusative exception.
aḥadun: delayed nominative subject and named exception base.

The sentence’s actual subject is aḥadun, not Zaydan.