As I understand the article we are all the children of Moctezuma in the broadest possible interpretation of the term because he has living descendants. Because he is a link in the human chain.
He was descendent from the sole human survivor, as well. It is not necessarily a direct linear descent, but an interconnected relatedness in which every living person on earth shares DNA with each other, and with persons such as Moctezuma, Cortez, the Hapsburgs...
The article went on to describe Brooke Shileds royal descent as an example. As if we really care about her personal lineage!
The author might have had a political agenda. He said every Arab has Jewish relatives, and vice versa. Words close to that, I don't have the article anymore. The analogy could equally be applied to conquistadores, and conquered.
I agree with you about the exceptions regarding the aboriginal people of Austraila, and others. I believe the entire premise is flawed. Yet it is presented as a mathematical certainty without addressing major issues.