Time that could be spent getting to a fallout shelter or to a place of safety is wasted by ducking down and turtling up. (None of which actually touches on my primary point that this Twitter rando judged the purported current threat by Russia to the United States by duck & cover drills he did as a child.)
We still do it for tornados and hurricanes but the left mocks it for explosions.
Comparing emergency response to a tornado/hurricane to emergency response to a nuclear strike is an utterly absurd category error to make.
Man, you are silly, why would you think that seeing a flash and diving for cover and protecting the head and eyes before the blast reaches you has ever been considered equal to having the time to find a nuclear bomb shelter and driving there or walking there if you are in one of those rare locations?
If you read about the injuries in Japan or any bombings or explosions you know that tornadoes and hurricanes share some of the same problems with them, flying debris, just living through some hurricanes should teach you that, I can share stories if you wish.
When the windows and building materials are flying at you, then duck and cover yourself, or if you prefer just stand there and watch as it comes in your direction.