If German is an ethnicity, I don't see why the US, which is older than the German Confederation (let alone the subsequent countries that have existed since then on that same land) has a distinct culture and set of shared values, cannot be.
But even that is too broad. It might be more accurate to get even more granular. For example, you might identify someone like Tim Walz as belonging to a synthetic Scandinavian-Midwestern ethnicity: although he has no actual Scandinavian ancestry, he grew up in Minnesota in what’s a recognizably distinct ethnocultural subgroup.
A far more useful analogy might be that “American” is a college football team.
"African Americans" certainly also separately underwent ethnogenesis, although the preferred nomenclature there has changed, and there wasn't really any disruption there. But I think it's certainly fair to count them as a distinctly and uniquely US ethnicity.
Europe the Germanic people have existed way before , a country is not tied to the ethnicity.
I’ve heard plenty of arguments about the German Volk as a distinct entity.
The argument was pretty decisively lost according to my grandfather.
Tell me, where does my Jewish German heritage fit in to Germany as an ethnicity? For some reason they didn’t feel very German when they left despite meeting all the qualifications…
That's a good point, and personally one of the reasons I disagree with the ethnic definition of "nation".
The other reason are of course the frisian, danish, sorbian, etc and many other similar minorities that have historically lived in the region of modern Germany.
I think the french definition (which defines the nation almost entirely around the language, not ethnicity or origin) is a much more interesting and useful one. Language determines who you can talk to, and what media you can read or watch.
There are many different distinct cultures in the US. Cowboys from north Dakota and Texas are both cowboys but have little cultural connection, and the hill billies Tennessee are very different from each.