The largest nuclear detonation set off by the United States was Castle Bravo. It was fifteen megatons. It had serious fallout issues, but it did not cause the kind of damage that is claimed for this weapon.
And supposedly, the original design was for 100 megatons, but they dialed it back because of fallout concerns.
I don't know, what you are referring to here, but to my opinion, given the distances of a relatively wide area with very limited population, the results where grave enough. Generations of the inhabitants where contaminated and had babies with many defects or deaths born. I guess the descendants of these poor people still struggle with the effects of this bomb.
Given such an event in or near a city of the US, I guess not only one city might get wiped out but the effects would be very real and destructive in a very big part of the US.