that doesn't mean that polynesia specifically benefits from the sea routes. Everyone else can get those trade routes as well.
Not if they can't leave their continent, they can't i.e. before they have gone far enough in the tech tree to be able to cross oceans. The point is that being able to set up those more lucrative sea trade routes so much earlier (remember that there will be more resource differences between cities on different continents) than every other civ could in theory give Polynesia a big early advantage in gold.