«ثُمَّ فَتَحَ كَرِيمَتَيْهِ»
Then he opened his two eyes.
Opening his eyes supplies a concrete counterweight to the earlier performance of blindness. The scene shifts from what al-Ḥārith had been led to believe to an observable act.
The act supports the conclusion that Abū Zayd could see then. It need not be inflated into a medical diagnosis covering his entire life; the narrative point is the unmasking of the deception.
Applied question
What kind of evidence challenges the earlier appearance?