Well, by definition the island metaphor is about the isolation that islands provide. So surely metaphorical islands are likelier to preserve the property of be

Well, by definition the island metaphor is about the isolation that islands provide. So surely metaphorical islands are likelier to preserve the property of being islands even if the physical ones turn out to be much more fuzzy. GP was clearly mixing metaphor and real to create an effect. "Nothing is really an island" -> "not even real islands are so isolated as it may seem".