I'd say that's true. While it's Dutch, and both groups have very little problems understanding each other and there's plenty of cross-influencing, I think the Flemish rightly identify themselves as a separate cultural group from the Dutch, with it's own rich and unique literary, musical and cultural achievements. So, I think you can speak of Flemish (for that part of Belgium) or Dutch (for The Netherlands), and if you want to refer to the combined thing, it'd be "Nederlandstalig" or "of the Dutch language" rather than Dutch.