There is no such thing as pure African. Everybody is mixed depending on how far back you go.
They may not have been horners at all. Those Africans may have lived further north because we had back migrations. The 40% west-Eurasian is so ancient you should not consider it as being mixed like Latinos or Afro-Arabs.
We are too ancient to be called mixed in the modern sense. Because if we are mixed, then every other group is also mixed. Btw, you wouldn't even know if the nilotes were pitch black, pigmys are very ancient and they are lighter than us.