I see your point, however I think, as was pointed out in the thread, not only is the terminology new, but the reason for the continued use of the term "fanfiction" is new as well, which is to say:
What is fanfiction vs. What is just regular old fiction?
Well, fanfiction by nature can't exist without the concept of intellectual property. It's fanfiction because, by modern standards or by modern law/copyright or whatever, the core story it's referencing is someone else's intellectual property, right? Not only is it amateur, but it's referential and amateur, and the key part here is that it also is using someone else's intellectual property as reference point. The idea that someone can hold for themselves a canon of narrative and characters and themes and whatnot and that theirs is the only True version of this story is a fairly modern phenomenon, as in last several centuries (this is a guesstimate so feel free to fact check me on that). Stories used to be told and told again by different people, whose audience then told it again to different people, et cetera, up until the early days of publishing of course. Every narrator would leave their own mark. These days, that's fanfiction, because we have a sense of intellectual property.
Of course, there comes a point even today where something isn't fanfiction anymore. Names, setting, plot, themes etcetera being changed to the point of being almost or totally unrecognizable. But that's not applicable the Divine Comedy, necessarily.
Which is to say, regardless of scope or artistic intentionality or merit or whatever, what makes something a fanfic rather than the broader category of an iteration of a story, to me, is a few things: the parent work being someone else's intellectual property, and of course facial similarity to the parent work in characters, setting, or plot, though that of course isn't really a part of this specific example.
The other side of this is, it would be historically inaccurate to refer to it as fanfiction. This has nothing to do with fanfiction's merits, as a modern concept. Just as it would be historically inaccurate to use terminology such as Leninist to describe the politics of someone who lived pre-Lenin, or to refer to a music piece as post-punk if it existed before that era (even with any apparent similarity) - it wouldn't make sense to refer to anything before the era of intellectual property and sci-fi fanfiction as "fanfiction".
Edit: for screengrab; relevant.
I am very sorry if I skimmed some replies wrong and someone already said this.