From a brief article from Answers In Genesis:
“Since the Bible describes all human beings as sinners, and we are all related (“And He has made from one blood every nation of men to dwell on all the face of the earth,” Acts 17:26), the gospel makes sense only on the basis that all humans alive and all that have ever lived (except for the first woman6 ) are descendants of the first man Adam. If this were not so, then the gospel could not be explained or defended.
Thus, there was only one man at the beginning—made from the dust of the earth (Genesis 2:7).
This also means that Cain’s wife was a descendant of Adam. She couldn’t have come from another race of people and must be accounted for from Adam’s descendants.”
for the full article, reasonably succinct - here is a link:
http://www.answersingenesis.org/articles/nab/who-was-cains-wife
They make the very solid theological point that only sons of Adam can be saved. There is only a plan of salvation for those who fell, who were Adam’s posterity.
I would, of course, agree with you. There is only ONE descendant from Adam. The point being, at least according to this article, is that homo sapiens were completely different, and processed information differently (perhaps divinely), than the neanderthals. It could be safely argued that those descendants, the descendants of Adam, made it through the Great Flood. The rest would have perished. And there wouldn't be a conflict with the text as the scriptures may be abscribing "the race of men" to homo sapiens.
The word ‘blood’ was added in Acts 17:26. And there are two different days of flesh beings created/formed in Genesis 1 and 2.