As a word of advice, never say that to a Dutch person.
Especially since 1940!
But it is a linguistic fact. It's not polite to call Austrians "Germans", either.
German and Dutch nationality were ill-defined at the time of the Dutch settlement of New Amsterdam. The Dutch brought in lots of German settlers.
In terms of linguistics they are not 'facts', one could easily say Dutch is a language but that Swiss German, Austrian German, High German and Low German are seperate languages as well. 'Tis all perspective and politics.
You are of course correct though; Dutch nationality does not begin to emerge until the foundation of the Dutch Republic in the 16th Century and then applying the concept of 'nationality' to it would have to wait for probably another two centuries.