>but the ruling class certainly shapes cultural/national identity through various means
Bourgeois nationalism is a part of the capitalist superstructure yes. However obviously a socialist movement and state would generate its own superstructure, and I think that one which embraces nationalism would be more effective than one which rejects it.
>though I'm not sure why culture is being equated with nationalism or the nation.
They aren't the same of course, but a sense of shared collective identity based around culture, history, values, and institutions is the core of nationalism.
>Why not just condemn the ruling class as parasites sucking the blood of the working class, and whose loyalties lie with capital rather than the workers?
We can do that as well, however at some point the national question is bound to come up. At some point reactionaries will accuse us of hating our country, at which point we can either confirm or deny that accusation. I think that denying it is a far more effective message, and if we deny it then we are compelled to reconcile our vision for the nation with our vision for socialism, and thus produce a left-nationalism.
>Nationalism is the obvious product of the nation-state, and since nations are not equal, different nationalisms will emerge.
You're right, the distinction isn't totally meaningless, and has important implications in all sorts of areas. However it does not in and of itself condemn all nationalism of the imperial core to be reactionary. The actual condition of proles in the core vs the periphery is not a qualitative difference but a quantitative one. They are both exploited by the same people and forces, the proles of the core are simply exploited less. The nationalism of the periphery is considered revolutionary by virtue of the fact that it is the nationalism of the exploited, but more specifically it is the nationalism of the great masses of exploited peasants and workers in the third world. It is not the nationalism of the comprador bourgeoisie and local collaborators with imperialism. If the masses of colonized countries can forge a nationalism which excludes members of their own culture who exploit them, then proles in the imperial core can do the same.