A special kind of 'fact': a social and cultural construct, such as a legal fiction that helps in the administration of the law, temporal fictions such as the days of the week, and geographical fictions like the Equator. Such constructs are part of life

So would Bill's "fictive island" be an island that it is for some reason useful to believe in, nuncle? Or is it enough that the island provides a location for a set of stories? That would appear a little self-referential and presumptuous - i.e. "If this book gets the huge amount of attention I think it deserves, the island will become as valid a construct as the days of the week or the Equator"

Perhaps the writer is trying to create a self-fulfilling prophecy (any vested interest in the book(s) doing well, perchance?)