يَجُوزُ فِيهِ الْوَجْهَانِ: النَّصْبُ وَالْبَدَلُ عَمَّا قَبْلَهَا.
Translation: Two analyses are allowed here: accusative and substitution for the preceding term.
With a non-affirmative complete clause (the exception base is mentioned), the book allows two analyses: accusative exception or badal (substitution).
«مَا جَاءَنِي أَحَدٌ إِلَّا زَيْدًا» — accusative.
«مَا جَاءَنِي أَحَدٌ إِلَّا زَيْدٌ» — nominative badal.
Their general meanings are close, but the syntax differs.