This is true, unfortunately the supremacy of American culture over the last century means that all of these concepts that do not fit very well into other contexts have been exported, I often run into situations in Denmark that would be horrifyingly racist in the U.S (not to say that Denmark doesn't have it's own racist traditions, but the grafting on of the American viewpoint does not exactly fit).
Of course when the concept is exported it in some way becomes the thing over time, and then people try to import the solutions to the problem.
Take what you read with a grain of salt. I'm certainly very skeptical of the implied claim that Germans wouldn't consider Greeks as white.
Note that being considered white doesn't preclude Greeks from being discriminated against, just like some people discriminate against e.g. white Poles.
I find it quite telling that you are surprised Greeks might be considered anything but white, while not showing any surprise about Turks. I don't think there's any difference in skin color, so that confirms my suspicion that "whiteness" isn't just about looks.
There are many more people of Greek descent than of Turkish descent in the USA. It's very possible that the person you replied to is simply not familiar enough with people of Turkish descent to express surprise at your claim.
"Whiteness" is a US concept, which doesn't make much sense in a German context. That sentence only tells me that some South Asians are neither white nor black.
I have no idea how they'd classify e.g. Turks. I think Greeks are considered white in the US, so maybe Turks too.
The other big immigrant group in Germany are Vietnamese, but they're kind of hamstrung by mostly having moved to East Germany. (East Germans in general are economically disadvantaged.)
I think statistics about ethnicity would be more useful if they were based on easily interpretable categories like own country of birth, country of birth of parents, grandparents, etc.