I checked the demo out, well I know what you are talking about now.
Yah I was wrong, half wrong.
But it would still be a better idea if you put walls at the side of those stairs.
This is because the zombies could probably use the parachutes to there advantage.
The fact that the zombies could do this( as in beat the humans like that) shows that the map maker made an error on this part of the map.
It is a big map so a mistake could have been made.
The map maker definitely did not account for parachutes.
The point is that if you enable parachutes on this map you will do more harm than good.
I guess you could use the parachutes as a quick fix.