Actually The -el in Satanael, as well as their role in punishing the false god Yalbadaoth suggest that Joker's ultimate persona is based on the Gnostic interpretation where they were an angel who rebelled against the Demiurge which is another one of Yalbadaoth's names.
But leaving that aside the term "guardian angel" is merely meant as a metaphor for personal protector, like the individual phantom thieves you mentioned