The form of our anti-nationalism is not against nations per se, it is against the cult of nations. We should desire strong states, as cultural experiments and as preservations of sanity that only a plural world can have. We should not demand entirely open immigration as a principle (even as we may choose to advocate for it in particular states), because as a diversity of policies creates consequences, those consequences should be felt by the societies that choose those policies (or fail to deal with problems), and proper solutions to misery require solving problems where they are rather than allowing escape. Usually. We primarily stand against patriotism of any kind, particularly but not only when it aims to serve only one ethnic group.
We speak in generalities on this matter because in politics there are no truly unique situations, at least in the essential traits. If we're to be consistent, any ideas we have for how to resolve or judge situations should be phrased in terms of universals, and we should cart our opinions made for one specific over to other similar specifics to see how they fit together; what details they may turn on, and how they help us reach whatever goals we have.