Although your question is about isolation, it is important to note that your text emphasizes the ways in which the various parts of Oceania were connected to one another by trade. We must pay attention both to the isolation and to the connectedness of Oceania.
Your text discusses the fact that there was a relatively great deal of trade and long-distance voyaging between parts of Oceania. For example, it points out that voyages brought sweet potatoes to areas that were very far from one another. It also discusses how the society of Hawai’i was changed by a second wave of voyages to those islands from Tahiti. Finally, it discusses how (for example) people of Tonga, Samoa, and Fiji traded and intermarried, creating relationships between all of these groups.
However, the islands of Oceania were far apart and travel and trade were very difficult. Therefore the islands were rather isolated. This led to their people developing different cultures, economies, and languages. Because islands could not trade easily with one another, smaller islands with fewer resources had smaller populations and less developed societies. Because they were so far apart, island groups tended to have languages that, while related to one another, were different enough so that their people would have a hard time communicating. The different island groups developed cultures that were different as well.
In these ways, the isolation of the parts of Oceania caused various parts of that vast region to develop in somewhat different ways.