Swamidass explains it in an interview:
"There’s been a lot of conflict about how science expresses its understanding of Adam and Eve. It has to do with misunderstanding the word
ancestor. We can understand it in the genetic sense, meaning someone we get our DNA from. Or we can mean it in a genealogical sense, meaning someone whose lineage we descend from.
"Genetics works in a very nonintuitive way. For example, my parents are both equally 100 percent my genealogical ancestors, and the same is true with my grandparents and great-grandparents. But my parents are each only one half of my genetic ancestry; my grandparents are one quarter; my great-grandparents are one eighth. Genetic ancestry just dilutes to the point where the majority of our genealogical ancestors pass on no DNA.
"Why is that important? Scripture doesn’t tell us about genetic ancestry. It does, however, tell us about genealogical ancestry. Historically, we’ve believed that Adam and Eve are the ancestors of everyone. We can ask: Does this mean genetic ancestors or genealogical ancestors? Well, Scripture can’t possibly be talking about genetic ancestry. It has to be talking about genealogical ancestry.
"That recognition really opens up an immense amount of space for theology. As Christians, we’ve had a lot of anxiety over what science is telling us about Adam and Eve. But these conflicts are based on what science says about our genetic ancestors. If we focus on genealogical ancestors instead, there might be far less conflict than we first imagined."
There are some reviews of his book on this Amazon page if you care to read them:
https://www.amazon.com/Genealogical-Adam-Eve-Surprising-Universal/dp/0830852638