EDIT NOTE: This question has not been satisfactorily answered previously.
Neither :
Fleishman's 'literary didactic ploy'
nor Williamson's 'no fully satisfactory explanation'
nor Blenkinsop's 'scribal error' (unevidenced) / 'could simply stand for Mesopotamia'
can be seriously regarded as 'answers' to this question. So I am hoping for a sensible answer to be forthcoming . . .
In Ezra 6:14 there are three kings mentioned :
And they builded and finished according to the commandment of the God of Israel and according to the commandment of Cyrus and Darius and Artaxerxes, king of Persia.
All three of these kings were kings of Persia, but in verse 22, the narrative states :
The Lord had made [the children of Israel] joyful and turned the heart of the king of Assyria unto them.
Who is this 'king of Assyria' ?