If you apply Seat 64 A's logic then you are offloading someone from Y even if you buy the last available F ticket on a flight which is full in J and Y. If a flight is full in Y then that usually means its already oversold . If you hadn't bought the F ticket then EK could have moved someone from J to F and another pax could be moved from Y to J thereby creating a space in Y for the poor soul who had to be offloaded.
Its all a bit like the butterfly effect at the end of the day.
EK routinely oversell , Ryanair and Easyjet don't do it as they are EU based and have to face stiff penalties.