Most peoples connected to sea aren't isolated at all, it's much easier to travel over sea than over land. I don't know the precise statistics, I think it's other way round -- morphologically complex and isolated languages tend to be found in mountain areas. I think J. Nichols is the strongest proponent, she has given examples from Caucasus...

There's a hypothesis that mountainous areas tend to preserve isolate languages better, and languages with complex morphology. Examples are Caucasus and the Himalayas. (Compare some languages in the Himalayas with related Tibetan and Mandarin.)

This doesn't mean these languages were formed there, but the isolation preserved some complex features.

(edit) grammar

But how do you know that the whole chapter 7 comes from a different source? What if only one paragraph comes from another source?