Sorry for the delayed response, but my work is busy now. It is a perfectly reasonable question, but its also something outside the purview of my day job.
For now, I will ditto this response from @john_harshman.
And direct you to the references and reasoning in my book, primarily the chapter on isolation. As far as I know, the argument there does not need to be updated in light of any recent scientific findings, at least not yet.
Note also the studies cited suggest that migration was rare, not that it was non-existent. All a recent GAE requires is rare migration, and may not even require any long range migration in Africa to work.