@Nymeria No one said you can't create a monoethnic culture. It's when you want that creation to be shown to the entire world, you may want to realize some people may question it, especially if it is all white.
And when the person who created the existing setting admits he wasn't sure if he should make a certain group non-white because he didn't want to make it seem as black=bad, but has no problem with PoC being cast in the world he created... why do you? (As an aside, if you're going to be so up in arms about people being cast that don't match the descriptions, maybe you should write your own setting and lore and monoethnic story?)
Also, there are totally isolationist monoethnic cultures/supposedly sovereign nations in the current world. We call them white supremist compounds, here in the states.