My C90GTi manual does not state that condition levers must be high idle, it does state RPM levers needs to be in high idle as the primary governor needs to have an underspeed condition with the propeller at the low pitch stop to command further blade angle decrease. With the RPM levers not fully forward and the propeller at the low pitch stop (oil pressure built-up) the RVS NOT READY light illuminates, indicating maximum reverse is not available. It has been over 10 years ago since I’ve flown the Kingair so maybe things changed in the mean while.
This is what it states for maximum reverse landing, no mentioning of condition levers, also not in the approach / before landing sections:
It would make sense to place condition levers in high idle if needing immediate engine response as the spool-up time is reduced, although I’m not sure how much this 10% difference in N1 actually speeds-up the process. We never operated the Kingair to aerodromes like that, as far as I remember we always kept the condition levers in low idle. In any case, it should not limit the maximum reverse thrust output, only the time to achieve max. reverse might be affected.