Well, SJ Baty, he does say he has "A couple of my DNA matches are descendants Edward Outlaw and Anne Ivey, so I suspect that I am also their descendant. " so it would seem based on that statement that he shares a common ancestor ...
But you are correct ... 2 parents, 4 grandparents, 8 great grandparents, 16 great great grandparents, 32 great great great grandparents and the farther back in time the lines are traced the wider the range of possibilities for his ancestry -- 64, 128, etc and which of those 128 ancestors is "the one" is not known
Edward and Anne were born (speculation?) in the late 1680's
I dunno, is this one of those cases where the x-DNA would help? Autosomal will not, but there's Y-DNA and mt-DNA ...
The problem with DNA test results is that there are no names, no profiles, to go along with it, unless the paper trail has been created and proven insofar as is possible. So he'd need to create profiles for parents, grandparents and so forth ...
Another issue frequently mentioned has been the composition of the genome pool his own DNA was tested against (compared with) -- from what others have said about the genome pools, he could have just as easily been "related" to some Polynesian or Mongol or Aztec -- granted, that's probably an exaggeration but it often comes across that way in the postings
From the standpoint of proven genealogy, he's best advised to create the paper trail with the evidence rather than spend any effort to "prove" he's a desc of this couple ...
Granted, it's sad that my understanding of DNA matters is limited to what I have understood of what has been said in G2G forum ... and I'm not wholly convinced that what has been said is reliable, since my motto is that: if it is not stated in terms that are understood by the many, it is because the person who is explicating does not wholly understand it themselves