If it helps, he's saying those two reasons are the evidence that the highlands were completely cut off, not that those are the cause of the highlands being cut

If it helps, he's saying those two reasons are the evidence that the highlands were completely cut off, not that those are the cause of the highlands being cut off. As opposed to an alternate history where there was significant trade or interaction between the highlands and coastal peoples. > The reasons why I believe that ... the highlands were functionally cut off is ...