Comparing JK to JC at 21 is a flattering comparison to JK because JC had a lot of value at 21. You seem to be comparing them at 21 based on production, but then the trade value of JK at 21 vs JC at 26. That doesn’t make sense.
Position labels are arbitrary, two players can be “wings” or “bigs” or whatever label you want and play completely differently. I struggle to understand the “tweener” label because JK can guard multiple positions and guys bigger and smaller than him. 6’8 super athlete is highly valued archetype. Since he’s able to guard multiple positions, it doesn’t make sense to call him a tweener. If offensively you think he’s strictly and off ball player that doesn’t handle the ball are do guard things, he can also defend that position. It just so happens to be the case that he can guard other positions. Unless you think he is literally a C only on offense, I cant see how he’s a tweener.
As far as what does letting him cook mean…it’s obvious. He’s already doing some with the ball in his hands, driving and passing. What people see in him is the ability to do even more of that because he has had some success with it and shown flashes. You act as if this guy has never bounced a ball or made a pass in his life.