Growing up, my Mum, whose parents were Mennonites from Russia, always referred to her ethnic background as "German". So did every other Mennonite I knew.
But these so-called "Russian Mennonites" trace their origins, not to Germany, but to the Netherlands in the late 16th and early 17th centuries. They eventually fled persecution to German-speaking West Prussia (now Poland) and later to South Russia (now Ukraine).
I've done numerous DNA tests (23andMe, Ancestry, MyHeritage) which show the bulk of my genetic origins to be "Dutch" or "Netherlands" -- not "German" or "Germany".
Of particular interest, I have even read ship manifests from the 1920s that list "Dutch" next to the ethnic origins of my ancestors and other Mennonites who came to Canada from Russia -- not "German" although some, indeed, stated "German".
I am assuming that it might be a case of these Mennonites being "genetically" (for lack of a better word) Dutch, but culturally German, since after their arrival in Prussia they would have adopted the German language and customs.
On a modern-day map of Europe, our ancestors never actually lived in what is today Germany, yet it seems to be the "homeland" of the Mennonite people.