وَيُحْتَمَلُ أَنْ لَا يَكُونَ زَائِدًا
It is also possible that the kāf is not additional.
On the alternative reading, kāf retains its normal comparative sense. The surface expression negates something “like His likeness,” but the intended meaning does not stop at that surface wording. Through indirect expression (kināyah), the wording is used to deny likeness to Him altogether.
Under this reading the syntactic construction genuinely includes kāf, while the emphasis of the meaning is understood through denying a consequence in order to deny what would entail it.