The island field is not physically an island. But it is surrounded by a dense swamp which make it physically difficult to reach. So in everybody’s mind who has ever tried to access they all accept it as an island. And interviews were a big part of Cartography at this time. Because the Indian Massacre of 1644 emanated from it, it was always included in subsequent maps even after they stop placing the Mankind Indians on it.
In the 1600s and 1700s the Island fields is always drawn as a real island. In 1865 Lewis Blackford has earlier maps with and island, yet his surveys shows none. So he tentatively draws an island with a light pen in his map even though none exists. The past over rode what he saw with his own eyes.