It's still possible to be plasma. Soviets were developing hypersonic aircraft utilising plasma right before the Soviet Union fell, but then it went nowhere, and the plans were sold to NASA at one point. But it sems that then neither NASA or US army were able to do anything with them.
I think Russians reviewed those plans again, and actually managed to create a proper plasma-accelerated hypersonic aircraft/missiles. This is probably what the Cs referred to as "missiles utilizing anti-gravity". Thus, it is possible for those MIRV "bomblets" to be coated with a layer of plasma to accelerate themselves (and while it's just my speculation, it wouldn't surprise me if that plasma also created a protective layer shielding them from potential intercepting attempts).