In a nutshell it's about genetic diversity. Subsaharan Africans include virtually all of the set of homo sapiens genes (save for mutations that occurred later on elsewhere), while populations outside of Africa have only select subsets of genes, due to the limited diversity in small migratory groups.
So if you're European, for example, nearly all the genes you have can also be found in the Subsaharan African population, but many of your genes can't be found in the East Asian population at all.
(I'm not considering Neanderthal genes at all, as those are a comparatively very small portion of genes.)
I'm not an expert in this and I don't have numbers here, but this is what I was taught in college anthropology at least regarding genetic variation worldwide.
That's true but I don't see how the earlier claim necessarily follows. It seems to be making assumptions around the amount of gene flow between Europe and Asia in the last 30k years, as well as that it was distinct migratory groups that seeded Europe versus seeded Asia.
It could well be true, it's plausible, I tried to google it without much luck, though.
From what I know it based purely on current measurable genetics, it's not making any assumptions at all. It's the genetic evidence that comes first that supports the hypothesis of distinct migratory groups, not the other way around.
to your point, a version of this i learned was that, if you had the magic ability to select two humans with the greatest genetic distance between them on the planet, those two people would both be sub-Saharan Africans.
So if you're European, for example, nearly all the genes you have can also be found in the Subsaharan African population, but many of your genes can't be found in the East Asian population at all.
(I'm not considering Neanderthal genes at all, as those are a comparatively very small portion of genes.)
I'm not an expert in this and I don't have numbers here, but this is what I was taught in college anthropology at least regarding genetic variation worldwide.