That way, the destroyer will only arrive if the destruction of the radar is false (ie, the player failed to destroy the radar)
Although Shade posted the better solution you've pointed out a common misconception in the way the is-event-false-delay SEXP works so I really ought to cover this.
In the situation described your solution wouldn't work. is-event-false-delay would evaluate to false so the destroyer would never appear. The mistaken assumption is that is-event-false-delay is the exact opposite of is-event-true-delay. It isn't. There are times when both is-event-true and is-event-false will both return false.
Basically the game works like this. At t=0 every event is in the state event-incomplete. They will stay in that state the entire mission unless they come true or reach a state where they can never become true. It is only at this time that is-event-false will trigger.
In the example you given the event would remain incomplete until the AWACS jumped out or was destroyed (or the subsystem was destroyed of course
). Once the destroyer has gone the game will realise that the event can never trigger and will mark it false.
When the mission ends every single SEXP that hasn't triggered is marked false. So these rules don't apply to SEXPs in debriefings.