A possible world is dominated by its own logic: it doesn’t need to adhere to the rules of the actual world (at least not beyond what makes it comprehensible for actual world-readers), but it has to make sense as a world.
When Matt Ruff wrote his novel in 2006, he was accordingly able to enter and rewrite a storyworld already known and shared by many. This resulted in the subverted storyworld the viewers encountered in the HBO series: a world in which the terrors of racism and segregation compete with the supernatural monsters in horror.