In the poetic illustration of taqsīm, “the donkey” and “the peg” are first named; then a distinct circumstance is allocated to each.
«هذا على الخسف مربوط برمته»
This one remains tied by its rope despite degradation.
«وذا يشج فلا يرثي له أحد»
And that one is struck, with nobody showing pity.
The point is specific assignment. The demonstratives hādhā and dhā distinguish the two referents so the respective states are not left unassigned. This fits the technical requirement of taqsīm.