I believe it was animated, possibly mixed with actual gameplay footage.

The thing I remember most is one Terran Marine appears holding a pad, and says "Just got an order from the general. 'BRB, Hot Pockets are done'". A second Marine asks what that means, and the first Marine sees a nuke or something descending on them and says he doesn't know. We then cut to gameplay footage of all of the Terrans dying.