German ethnicities existed before and outside Germany, it is not the same as in the US, where Americans didn't exist before it was founded. In Europe peoples' ethnicities are not tied to the state the same way as it is in the US.

For example, Volga Germans (https://en.wikipedia.org/wiki/Volga_Germans) were Russian subjects who lived outside of Germany; Baltic Germans as well.

More examples - Hungarians in Romania, Turks in Bulgaria, and so on.