The crazy theories about the 2ch owners coming up with the story to pay off lawsuit fines seems much less credible than the story in the posts themselves. They're great posts, and there's no denying the community enthusiasm behind the man. It could be just elaborate fiction he decided to write in a forum instead of a magazine article, but if you notice, it's kind of a boring story. There's no real conflict. It has a real anticlimactic ending that's satisfying because it's so authentic and relatable.
Note: I'm refering to the original posts, not the dramatized dorama.
If it's a fictional story, it's a really weird piece of fiction. I think the forum would catch on if it started sounding like fiction. The bit about Densha sitting down to watch that shoujo anime (what was it called? Perm-something?) and not being able to register or concentrate on it at all after the traumatic confession he undertook, is just too good to be fiction. Only the best fiction writers are able to come out with stuff like that.