وَقَدْ يُقَدَّمُ الْمُسْنَدُ إِلَيْهِ لِيُفِيدَ التَّقْدِيمُ تَخْصِيصَهُ
Sometimes the subject is fronted so that the fronting conveys its special restriction.
Here takhsīṣ concerns identifying a predicate or action particularly with a given referent rather than with a competing referent. The effect requires an appropriate construction and context.
«Zayd came» has a fronted subject, but it does not invariably mean «Only Zayd came». Al-Jurjānī's discussion is tied to the specific grammatical configurations and circumstances in which restriction is conveyed.