ثُمَّ قَدَّمَ فَصَاحَةَ الْمُفْرَدِ عَلَى فَصَاحَةِ الْكَلَامِ وَالْمُتَكَلِّمِ لِتَوَقُّفِهِمَا عَلَيْهَا
The fasaha of the word is placed before that of discourse and speaker because the latter two depend upon it.
Discourse is made from words; a decisive defect in a constituent can affect the judgment on the whole. A speaker's fasaha involves the ability to produce discourse of the relevant quality.
The sequence word → discourse → speaker therefore tracks a dependency in the definitions, not merely an arbitrary arrangement of chapters.