Schools still teach DD.MM.YYYY. That’s what’s relevant because that’s what people actually widely use.

Shouldn’t Americans be very familiar with the idea that official norms (the metric system) play no role if the people don’t want to use them?

I understand - it seems I was too optimistic to read that the use of yyyy-mm-dd has become "the official standard" in Germany, without considering the popular usage and convention under discussion.