According to the Quran, he's not an atheist, but a kafir. One that knows the truth and works to suppress it.
Like Satan, who says to man, ‘Do not believe!’ but when man disbelieves, says, ‘I disown you; I fear God, the Lord of the Worlds,’ 59:16
Salam alaikom brother, wa rahmatallah wa barakatu
I've thought about this verse before but thanks for bringing it up. It means he causes people to disbelieve in reality of Jinn, Angels, power of Prophets (a) while he witnesses all that. His fear of God lord of the worlds, is fearing the order of heavens and earth. From another sense, he only deceives himself while he knows God is real.
Same when during battle, he fled because he feared God. But in reality, he doesn't believe in God as far as he tells himself. He fears the order of heavens, and so fears the God of the Angels and God's forces, but doesn't really believe in him as being real. From another sense, he knows the signs of God point to God and is only lying to himself while knowing he is real.
The thing is Iblis calls to himself, but sometimes in that call to not fear punishment from God and Angels and his chosen Authorities and forces, people go too far and deny all that, They become blind to all that due to the call of Iblis. But he himself is aware of magic despite him causing people to disbelieve in the magical worlds. So here he fears their Lord as in the command of order of heavens and earth, is based on belief in God.
Sometimes God talks about true intention of a person, for example, he says Pharaoh and his chiefs with him were SURE of God's signs from Musa (a) but denied them anyways.
Iblis has deceived himself he is a good guy. When you realize he actually believes in dirty acts and sexual deviance, and has tricked himself that he is a compassion ally to even humans, you will better to witness the Kaheneen and their ways, and you will understand how Devils think, and be able to guard against them.
Although many or most kaheeneen believe in a Creator or claim to, there is an elite of Magog who are followers of Gog, who are told the secrets and are told about the destroyed cities and nations, and they are preparing for the final show down as well, and Iblis does believe he can overthrow heavens and earth as far as he deceives himself because he stills grows in power and his magic increases in power and so do his forces. However, they can't come near any of the power of the Prophets and so are ever foolish, like the Pharaoh when he wanted to look a the God of Musa (a). He thought (he tricked himself) that can he reach the power of Musa (a) eventually.
The faith Pharaoh had about "God of Musa" that he can overthrow it, this is same faith of Iblis about the God of Ahlulbayt (a), he believes it's their delusion. From same way Pharoah feared Musa (a) and his prophecies, Iblis fears God... but he doesn't believe he is real.