I think the dissonance you’re running into here is that places like Reddit are not monolithic. Saying Reddit, basically a very large collection of user submitted writing, is _entirely_ non-fiction is not correct and ignores years of short stories, web serials, fanfiction, role playing, story telling, and all manner of creative fictional writing. I’d wager to say the bulk of such work on the internet exists under the Reddit umbrella, or is at a minimum cross-posted there.

Many items on those are fiction. Some even intentionally so.