There's no reason that the Nationals need be North Korean. There is diversity of political opinion in the South and not everyone loves Japan or has forgiven them for the long and brutal occupation. The film could easily be seen an attack on Korean sovereignty for any partisan of a unified Korean State. That a Japanese corporation is behind it certainly doesn't help.
On the other hand, historically the view of Japan as the important enemy is more strongly associated with the North. Fighting the Japanese occupation was probably a more important part of Kim Il-sung's political life than socialism and much of the divide between North and South was based on collaboration with occupiers than on a love/hate relation with capitalism...most Koreans in both countries at the time their civil war was hot were agrarian peasants.
On the other hand, historically the view of Japan as the important enemy is more strongly associated with the North. Fighting the Japanese occupation was probably a more important part of Kim Il-sung's political life than socialism and much of the divide between North and South was based on collaboration with occupiers than on a love/hate relation with capitalism...most Koreans in both countries at the time their civil war was hot were agrarian peasants.