Actually ...
In some past versions of EAW or OP (don't recall which at the moment), sometimes you could get a Victory (not Astounding Victory) by destroying all the defenders and then leaving the map. However, I think that might have been in base assaults, and may have had something to do with the mission victory awards. I wouldn't recommend trying it with a planet assault in a nebula.
This sounds like something Dave "Nuclear" Wessels (the Evil one) should take into account for his custom missions, though. Oh Dave ...