> a 1,000 metre climb over about 30 km takes you straight up onto the edge of the eastern highlands
i.e. because the Markham River provides a pathway from the Lowlands into the Highlands.
The paragraph you quote concerns a different proposition, as may be clearer if some of the subordinate clauses are elided:
> The reason why I believe that […] the highlands were functionally cut off, is that […] they themselves did not ever speak of or show knowledge of the highlanders. Similarly, when the highlanders first encountered Australians, they had no concept of the coastal people, or where the shells came from, or any further concept of the broader world.