I honestly don't understand in what sense they are not fans creating derivative works in the same fictional universe.
To be sure, that's not what we usually mean by fanfiction, because part of the connotation of the word is that it's done by a modern "fan" (which has connotations of its own) writing about modern fiction. But I don't see a substantive difference.
To be sure, that's not what we usually mean by fanfiction, because part of the connotation of the word is that it's done by a modern "fan" (which has connotations of its own) writing about modern fiction. But I don't see a substantive difference.