For a tornado you at least had a fighting chance. Whereas, a nuclear bomb will not be deterred by squatting down on the floor of an open hallway. Even under ones desk was more protection than that pose was.
I get the joke. Cute! Just as a comment on the picture itself, those of us that lived through those cold war days, it is amazing just how ridiculous that whole procedure actually was. No wonder we grew up thinking our parents (or elders generally) were quite stupid.