Was Jehoshaphat King of Israel?

No, it is not a mistake. It is an example of the flexibility of biblical language.

The expression "king of Israel" can be employed in either a specific sense (of a certain lineage) or in a generic sense (nationally). More technically, Jehoshaphat was a king of Judah, i.e., a king over the two principal tribes (Judah and Benjamin) that composed the southern kingdom of Judah. However, since the entire nation was "Israel" in a more general sense, it was not incorrect to speak of Jehoshaphat as "king of Israel."

There was likely a spiritual reason for that designation in this passage. C. F. Keil noted: "Jehoshaphat is called king of Israel instead of king of Judah, because he, as king, walked in the footsteps of Israel, Jacob the wrestler with God, and was a true king of God's people" (1978, 395).

Whenever there is a reasonable way to explain an alleged conflict between historical statements, that explanation should be considered, rather than frivolously charging the sacred text with a discrepancy. A genuine contradiction exists only when there is no possible way to reconcile accounts that appear to disagree.