Of course it is possible to take this verse at its word and accept that Corihor actually changed his heart and Shule really forgave him. It is possible that it happened just that way. If it did, however, it would be extremely rare in the history of political conflicts in the world. Western history is replete with kin who are murdered to protect a throne that they had not even yet threatened, but might in the future.
The most likely scenario, historically, is that Shule was unable to completely defeat Corihor, and that Corihor retained rulership in his land, even though he had lost control of Kib and therefore the larger area. It is important from a historical perspective to note that Corihor retains “power in his kingdom.” Shule may claim some control over Corihor, but Corihor retains power, and therefore a powerbase, which his sons will use.