"you can't instant KK a J seat if Y is oversold"
Not quite accurate but close.
You can't instant KK a J seat if the whole plane is oversold. I have gotten awards where Y is oversold just a bit but not enough to take up all the seats in the plane.
e.g. 150 seats on plane, 134 Y 16J, 136 seats sold in Y and 4 in J means whole plane is not oversold, as an SE you could instant kk your J seat on that flight.