Well, two comments on that. Firstly, I did say at busy times. Secondly, slots can be constrained by terminal or stand capacity, not just runway slots. Terminal capacity is not applicable to freighters but apron and stand occupation for longer durations can be, particularly with the TP work going on.
But as I mentioned, perhaps flexibility during the covid crisis might have been an option but the airport presumably assessed financially it wasn't worth it. It's sad in my opinion for the reasons given by others, but it doesn't look as if a change of policy is likely.