You are correct he is not a man but he was not an angel/messenger. He was the anointed cherub and was adorned with jewels very similar to the high priests urim and thummim.
If being adorned as a priest who was he a priest for?
Why did he have a throne?
What man has been cast out of heaven?
What man has walked in the midst of the stones of fire?
It is very clear that he was cast out before the recreation story.

When I need expert advice I tend to talk to myself
The older I get the better I used to be