While in theory a slot pair could be used by any sized aircraft, the reality is often different. A pair which gives a 45-60 minute on ground time can easily be used by a F70, it cannot be used by an A380.
You also need airport infrastructure. You cannot have 40 A380s on the ground simultaneously at most airports (except DXB which has the infrastructure).
Slot allocation is not as simple as it seems.
Note that the slots have only been lent to ABC, KL retains the right to them. They could lose them, however, if ABC again fails to use them 80% of the time.