You just said, “trace back equally to sub-Saharan Africa and to Europe.” It’s awfully silly to say that sub-Saharan African ancestry and European ancestry are real, but black and white are not more or less the same things.

So they default to assuming that population density is constant all over the globe (including, hilariously, the oceans), so therefore it’s only logical that nobody could possibly draw a line between one race and another. Races must just constantly bleed into each other in smooth gradients.