Sure, but it sometimes is useful to remember that those fictions are indeed fictions, and can be changed. Doesn't mean they are not useful fictions. Doesn't mean they should be changed. Saying they are meaningless because they are fictions is, of course, absurd. Civilization is built pretty much exclusively of useful fictions. But is good to remember we can change the set of fictions we collectively believe in.

Think about rights and legal codes. When the law supposedly comes from god, like it did in many ancient societies, it makes no sense to ask whether it is just or unjust, as it can't be changed except by an act of god. When you recognize that law is human made, that doesn't mean it holds no power, but it means humans can change it, and unjust laws can be repealed when enough humans agree they are unjust. So is with nationality. Is not a fact of nature, is a distinction we agree on because it serves a purpose. Is that purpose necessary? Is it just? Is there a viable path towards changing it? Maybe, maybe not. But acknowledging it is made up allows us to at least ask those questions.