In «جَاءَ الْقَوْمُ لَيْسَ زَيْدًا» and «جَاءَ الْقَوْمُ لَا يَكُونُ زَيْدًا», Zaydan is accusative as the predicate of the defective verb.
Laysa is a defective verb; its subject is an understood pronoun in this analysis, while Zaydan is its accusative predicate.
Do not attribute its case directly to the particle illā, which is absent here.
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Al-Mustathnā: Connected and Disconnected Exceptions, Four Case Rules
In «جَاءَ الْقَوْمُ لَيْسَ زَيْدًا» and «جَاءَ الْقَوْمُ لَا يَكُونُ زَيْدًا», Zaydan is accusative as the predicate of the defective verb.
Laysa is a defective verb; its subject is an understood pronoun in this analysis, while Zaydan is its accusative predicate.
Do not attribute its case directly to the particle illā, which is absent here.