"Of course, the whole point of a Doomsday Machine is lost if you keep it a secret," said Dr. Strangelove to the Soviet ambassador in Stanley Kubrick's classic film of the same name. Fifty years later, it would appear that the Russians have finally watched the movie.
In Kubrick's film, a rogue American air force commander orders a nuclear strike on the Soviet Union — but he doesn't know that the Russians have built a Doomsday Machine that will automatically explode and spread lethal radioactive contamination all over the world if American nuclear weapons hit the USSR. So everybody dies.
Moscow doesn't want the United States to make the same mistake in real life, so it has just let us know that it is building a mini-doomsday machine. It wouldn't destroy the whole world, just a half a continent or thereabouts — like, say, all of the U.S. east of the Mississippi River, or all of China within 1,500 km of the coast.