The original text elaborates the case as follows:
ففي البط يصدق كل منهما، وفي الفرس يصدق الحيوان فقط، وفي الثلج والعاج يصدق الأبيض فقط،
Both are true of the duck; only “animal” is true of the horse; only “white” is true of snow and ivory.
The intended duck is a white duck, and the horse is one that is not white. Ducks are not invariably white and horses are not invariably non-white. The logical point is that appropriately described examples exist.
Accordingly, the extensions meet in some instances and diverge in others: neither equality, disjointness nor one-way inclusion applies.