They mentioned that if there had been a result with low damage, this would make the Chinese and Russian less afraid of the bomb, and the US would lose face. I haven't found any source corroborating this statement.
This is stated in the full online video at 1:19:48
http://www.pbs.org/program/bomb/
The mention is brief, with the quote coming from Richard Rhodes, who's written well-regarded popular accounts on the development of nuclear weapons. A somewhat more detailed take can be seen in an interview with historian John Lewis Gaddis at
http://www.pbs.org/wgbh/amex/bomb/filmmore/reference/interview/gaddis5.html
He attributes this to concerns Paul Nitze had about the use of nuclear weapons in Korea.
I suspect this is a bit of a case of complexity getting lost in within the constraints of a relatively short tv documentary. It's worth considering the sort of things military planners, policymakers and executive decisionmakers were grappling with at the time. What is the nature of the Soviet/Communist threat? Are they, in fact, the same thing? How should it be handled, in either case, as a matter of policy? Or in the case of a specific crisis, as in Korea? What should the US nuclear posture and strategy be, in general or specifically? What are the inflection points between 'resolute response' and 'dangerous escalation'? In light of that, it's perhaps somewhat overly simplistic to say the US planned to use nuclear weapons against North Korea and then just decided not to because mountains.
If you're asking about the physics of it I would go to /r/AskScience
If you're asking about if that's a reason not to then you're probably in the right place.