Elon Musk, the billionaire founder and CEO of the private spaceflight company SpaceX, wants to help establish a Mars colony of up to 80,000 people by ferrying explorers to the Red Planet for perhaps $500,000 a trip.
(...) we've put all our eggs in one basket. If we were on many worlds and were to mess up down here, there's a way for the human species to continue. I don't for a moment propose that the Earth is a disposable planet, and we have to put enormous efforts into making sure we don't muss up down here. But there is a chance.
The point was less "Rar EMPs" (I imagine you could construct nanites that would survive an EMP) and more "Rar broadly tailored weaponry." I can't imagine exactly what form that weaponry would take - maybe anti-nanite-nanites or even biological parasites which strip the nanites of usable materials. Heck, maybe even just emitting audio waves at the resonant frequency of the nanites. The point is that we could tailor and utilize weaponry on a broad scale which has a minimal impact on us.
Whereas we can't (yet) design viruses that wipe out only, say, Russians, to go back to the cold war - and even if we could there would be a short mutational gap to targeting everybody.