A black German born and raised in Germany with German traditions and with German culture is ethnically German.
Ethnicity has a lot more to do with culture and place of origin than anything else.
Dev patel for all accounts is ethnically british, he is a second generation immigrant whose parents immigrated when they were teens to london, that is a long time.
As a side note he was very good at taekwondo.
There is a consensus about ethnicity, you just don't agree with it. It is more about culture than race, as an example look at the Jewish ethnic variations. They are all Jewish, they all cut their pipi, but they span lots of races.