True, but that depends on whether you would consider a tribe and a nation as coterminous.
Anyway, this discussion seems to overlook the strength of non-national identities, (to Christendom, or to your lord, or anything in between). If nationalism is the ideology that says that the loyalty to the nation should always be above all other loyalties, then it seems the way to bring about nationalism earlier would be to weaken all other recipients of people's loyalties. And conversely, the way to delay nationalism, would be to reinvigorate those alternative loyalties.