Yea, as if Americans are running around Germany stopping strangers and screaming, "I'm German!" at them... The culture leaves Germany with the people who move to America. If your grandparents were from Germany then your parents certainly would have grown up hearing the language, eating the food, and being exposed to the culture. Small things like parenting styles, attitudes about work, life philosophies, mannerisms, nationalistic pride, and other things inherently German don't just disappear when they cross borders.

In America people naturally form friend groups around people with similar ethnic backgrounds because there is often a greater correlation in family culture which feels natural/comfortable when making friends. American kids of Indian decent for example typically have strict parents who demand they do well in school, grow up eating curry, share religion, are interested in Indian people of the opposite sex, have homes decorated similarly, have grandparents who act the same etc.. One glance at an American cafeteria would illustrate the point. All the kids of Asian decent sit/socialize with each other because even though they were not born in an Asian country, they share similar views, home lives, and cultures which are passed down in their families. People also seem to be romantically attracted to others with similar physical characteristics. I don't feel like I need to go into detail as to why or give examples but the externality of this is that it is extremely common for American parents who met in the US to be of a similar ethnic background.

There are defining features of someone from a specific country. You can look at an American person of Japanese decent and tell they weren't akin to the Vikings. When you grow up in America it is a common talking point to ask someone what their heritage is because it says a lot about the way you were raised and gives insights into what they were like. Obviously your ethnic background isn't worth talking about in Germany where 95% of the population is either German or Northern European and the question holds no weight as a topic of conversation.

The point being when an American of German decent visits Germany it's a very surreal experience. It's expensive to visit so those who choose to usually are familiar with the culture, language, and history to some extent. It's the first time you are surrounded entirely by people who look like you and there is a faint sense of familiarity due to the culture that was passed down in your family. The guy in the video appears to have a weak grasp of social norms so I'll give him as pass. But as an American visiting Germany you are inevitably going to end up talking to someone there. That person will ask where you're from and why you're visiting. You will explain that you're of German decent because in America that would be the end of the explanation. We aren't expecting a cookie, high five, or some sort of praise you pretentious fuck.