Well it also depends on what type of processing each unit requires:
Example, if each unit had a complex AI system, which was fully automated and such, the lag would be worse than if it was just "go here, shoot here".
There are TONS of things taken into account when determining lag, other than just polycount and textures. The game engine makes a huge difference. I'm just saying judging by the looks of that game, it would take a lot more to bog a computer down than say, 500 units on Warhammer or AoEIII.