This statement is another in Ether that has the feel of manipulated history. A king is held captive by a rebel son, who thereby becomes king. Another of the father-king’s sons captures the rebel brother, and the father-king bestows the kingdom on the rescuing son.
The capture was never one of complete incarceration, since Kib fathered a child after his capture. After Shule “rescued” Kib, Kib declines to rule and instead bestows the kingdom on Shule. While it is possible that Kib was grateful, dubious about his ability to rule, or unwilling to take up the tasks of ruling in what must have been old age, it seems more likely to me that Shule simply dethroned Corihor and continued his father’s de facto imprisonment. Apparently, controlling the king’s person was a legitimate claim to political power. Since the royal descendancy goes through Shule, not Corihor, Shule’s actions are presented in a favorable light, while Corihor’s are negative. But probably they were the same kind of action.