you have to go back further in history than americans are accustomed to admitting is relevant, but the timeline is longer in east asia, and the north of korea has indeed been a part of china, although the south never has. trump may or may not have fully understood, but the chinese were no doubt presenting the perspective that they should have control over all areas that have historically been in the chinese orbit; this is a perspective they have presented repeatedly, over many years. and, the north of korea is legitimately in this orbit. it is for this reason that korean unification is not inevitable.