It's actually pretty interesting technique that they used. They don't just clump up the mutas. From the results page:
Contemporary StarCraft wisdom tells us that the best way to use mutalisks is to clump them. In human versus human battles, this makes it difficult to single out the weaker mutalisks, because the units are stacked on top of each other. However, UC Berkeley’s team identified a flaw in this tactic; it reduces the damage output of each individual mutalisk, because not all mutalisks will fire when using this tactic. Instead, they employed a model in which mutalisk are always moving, maximizing damage output while simultaneously maximizing movement.
Video can be found at the bottom of the page.