I am disappointed at the snarky tone of this post but I also think that those commenting here should engage the arguments of the book if you are going to raise such objections. There is a chapter of the book devoted to this topic. Below is a quote from the beginning of that chapter, after which Swamidass discusses the implications of isolation in detail.
“What if this assumption is incorrect? What if one or more populations were isolated for thousands of years in our past? No universal ancestors could arise except before or after that time of isolation. If the isolation was for a very long period of time, this could substantially push back when universal ancestors would arise. What it would mean if a specific human population was, in fact, isolated? The most likely candidates might be the indigenous populations of Tasmania. If this population were isolated from six thousand years ago till AD 1, would this be a problem?”