Well, first of all this is a pretty convoluted way of re-creating the sprEnemy.Count expression! :)
However, note you 'Destroy' the instances then try to count them. You're taking advantage of a quirk where Construct 2 does not really destroy objects until the end of the tick, so without the 'Wait' action it can count them correctly.
Also note from the Wait tutorial:
ne more trick: "Wait 0 seconds" postpones the following actions until the end of the event sheet.
So if you add "Wait 0 seconds", it does not ignore the condition - it will postpone the subevents to run at the end of the tick... after Construct 2 has done it's end-of-tick actual destroy of objects. So there are no objects left.