I think what he is trying to say is that when summoning a building the summoning unit is displaced to the nearest free location because it finds itself embedded inside the non-pathable building it just summoned. Since the game does not make much distinction between cliffs and non-pathable buildings it displaces him up a cliff where he has an unfair advantage in the map.
One solution would be to fill the top of cliffs and other pathing islands with npn-pathable pathing. This means that units cannot be displaced up there because there is no free room at the top for them. If it fails to find an area to displace the unit, the unit will become stuck where it was, which might be better than it going to where it should not be.
Another solution would be to enable builders to walk through builldings and hence not be able to be displaced by them. Thigh might be possible by using different pathing layers, such as water. Honestly I am not sure.