oh I agree, if a military attack/emp were to occur we’d use our mutually assisted destruction weapon systems (military hardened bases, not susceptible to emp) & wipe them out. it’d be the literal end of the world over there.
yes, it was very optimistic. 5 to 10 percent of the current population of America (the rest of the world wouldnt matter at that point) is still 15 to 30 million people. even spread out there’s still a good chance to develop small kingdoms, and those would specialize as they always do. trade routes would organically develop over time. it’s true that we do get a lot of things from overseas but the North American continent has loads of untapped resources, it’s just cheaper to get them from somewhere else in the world right now (take coal for example over in west virginia - they’d specialize in coal extraction, processing, chemistry, industrialization).
personally, I think the climate would stabilize fairly soon after such a catastrophe, probably within just a few generations - and if not, so what? no one is going to be living in cities that are underwater anyway.