Improve your least favorite Final Fantasy.

[Sephiroth]

The real issue is that Episode Zero was a Japanese only novella/web series.

What are you talking about? Episode Zero is a book that exists in other languages. I even own it in German. And the book shows a alot about the characters and previous incidents.

That's the excuse of a weak writer.

Not in the slightest. Sometimes, if you want to write something in a certain way you just have to sacrifice something. Otherwise you would have to write the story in a way that you do not want it to write. And an author fully has the right to decide how exactly he writes the story. If he can do that by then adding details that he thought of anyway, I have no problem with that. Authors often have to sacrifice something to write a story the way they want. Just because you do not like it the way it is written because you would have to know addtional details through something else does not make it "badly written". I watch a lot of shows that sacrifice a lot of logic by doing stuff like time freezes just so they do not need to worry about other details but I do not call it bad writing because I understand what exactly it is for.

Frankly, what I've read of it is not even that good

I am glad that you do not take your opinion about something that you hate in the first place and use it as factual statement.

and doesn't help that much for the narrative.

It helped me a lot to immerse and understand more about the characters and what the background of the story is.