Harris et al. 2023, “Diverse African genomes reveal selection on ancient modern human introgressions in Neanderthals” https://www.sciencedirect.com/science/article/abs/pii/S09609...

“… most Neanderthal homologous regions in sub-Saharan African populations originate from migration of AMH [anatomically modern human] populations from Africa to Eurasia ∼250 kya, and subsequent admixture with Neanderthals, resulting in ∼6% AMH ancestry in Neanderthals. These results indicate that there have been multiple migration events of AMHs out of Africa and that Neanderthal and AMH gene flow has been bi-directional.”

This is, admittedly, research published after Slimak’s book came out, and referring to an earlier time period.