I think an important point (that they make) is that you can write an app/project using django-nonrel and have it be completely compatible with the vanilla Django on another server.
i don't think it's really a fork, but rather a django database engine that supports "datastores" + related tools
Unfortunately right now it is a fork -- django-nonrel makes some internal changes to Django. I'm hoping to reconcile those changes with core at some point so that -nonrel could indeed be just a backend, but we're not there yet.