Interpersonal concepts only work when everyone involved in the connection supports them — and geopolitics is interpersonal relationships between two metapersonal entities. The rebels merely show that the concept is a conceived one, not that it's a "myth". "Myth" implies that the story we tell ourselves is not just real — they aren't supposed to be.
Sovereignty of state, like autonomy and independence of a person, works until someone intrudes upon it. Does it mean that one's autonomic existence is an unreal concept? No. Just see how you feel when someone you don't know leans forward to you a few inches closer than you would like, and you'll see how real it is.
What do you do when someone doesn't respect the same rules as you? Now, there's a question.