EMP is a line-of-sight weapon when detonated in space. 1MT 400 miles over Kansas takes out the North American power grid coast to coast. Period. Lower yield weapons are more effective than very high yield weapons. That 50MT weapon was not detonated in space. Russkies like their electricity.
When Czar Bomba was tested, the Soviets didn’t have the capability of putting anything that big into space. They had a hard enough time just dropping it from a specially modified tupolev bomber and getting away in time.