A people and their language predated the concept of nation-states, but when the latter arrived obviously (geo-)political interests started to blur the facts.
So if you conflate the German state with Germans (I'd challenge that and view the German state as a continuation of the Prussian state), and you don't like the interests of the German state, it is predictable where you'll land on this issue.
Because of this, even if their national anthem does so, calling the Dutch Germans would infuriate them and rightly so, because it would imply justification to some for things like those happening between Russia and Ukraine right now.
I think in the end it is also a matter of "national" self-confidence. While Luxemburgish is virtually indistinguishable to the German ear from say the dialect of Cologne, Swiss-German is hardly understandable for anyone outside of Switzerland. Yet, the Swiss don't have an urge to re-label their dialect as a separate language. And the urge of the Dutch to re-lable themselves is lesser than that of Luxemburg because seemingly no one questions their identity.