It seems more likely that a nuke could be smuggled into the United States across the southern border, and detonated in Dallas or Chicago, or maybe detonated on a boat in a harbor near DC or LA. Or, maybe an EMP strike like in the book, One Second After (launched from a container ship off the coast which is then sunk immediately after launch).
And what if a limited strike like one of those above occurs? Does the United States launch a full scale nuclear attack on Russia and China?...triggering them to launch a full strike against the United States? Maybe. But probably not.
The most deadly is a full exchange. But there are more likely scenarios that are survivable with the proper knowledge and preparation. And, focusing only on the most deadly but least likely scenario does a disservice to the American people. There are ways to survive. But, this analysis requires nuance.