كَانَ مِنَ الْجِنِّ فَفَسَقَ عَنْ أَمْرِ رَبِّهِ
He was of the jinn, and departed from his Lord's command. (Al-Kahf 18:50)
The verse directly states his kind of creature. Presence in an angelic assembly does not establish angelic membership, whereas the express Qur'anic designation of Iblis as a jinn identifies his category.
It also describes his refusal as disobedience to a divine command, not merely an involuntary misunderstanding. Thus his nature and act must be identified before using the story as a counterexample to angelic protection.