An alternate solution, discussed in the book (and curiously omitted by Jay), is that they decide “nearly universal” descent is sufficient, and have a good case for why Tasmanians have human worth and dignity regardless. Remember, theology does not speak with scientific precision. With this in mind, there is no reason to doubt that Adam and Eve, if they are real, are ancestors of (nearly) everyone, even if they lived only 6,000 years ago.
Of course, if nearly universal descent is not sufficient, your analysis holds. I’m just not sure “non-realism” is precisely the right term. Let me think about that.
In the end though, your response is strong. In particular: