In the poem, «تَظُنُّ» (“she imagines”) and «أَرَاهَا» (“I see her”) both involve mental assessment; that affinity can tempt a coordinated reading. Yet the speaker does not intend to present his own conclusion as a part of the woman’s assumption.
Reading «أَرَاهَا فِي الضَّلَالِ تَهِيمُ» as an independent sentence clarifies that it is the poet’s verdict on her thought. The selection of faṣl rests on guarding the intended predication against ambiguity, not on ornamentation alone.