Now if you contend that ancient extra-terrestrial aliens took them that far ...
“Humans were not then capable of sea navigation without sight of land for weeks.”
They didn’t have to be... All they had to do was jump in the water on a floaty and the circumpolar winds and currents would take them right along Antarctica to the tip of South America.
No need for complicated navigation at all. And they could survive the trip by hugging the ice pack of Antarctica where there is abundance of food and water.
If the pre-Thule could do it they could do it. Ice pack travel and survival ability is not uniquely possible to the north Arctic.