And it doesn't come from non-African Admixture
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3845641/
http://evoandproud.blogspot.com/2012/02/were-there-neanderthals-in-africa.html?m=1
Paabo simply assumed that sub-Saharan Africans had no Neanderthal admixture.
He used their genome as a benchmark for zero archaic admixture.