This isn't correct. It is infringement, for example, to write Harry Potter fan fiction in private on a typewriter, even if another soul never sees it. Copyright includes creation, not just distribution
If you think about it, writing “Harry Potter” on the internet could be infringement because those words might be in the book, and most worrisomely you are inducing people to make “copies” of the books in their minds. There’s no way to calculate what you owe Rowling from this post, it could be infinite.
(Thankfully I’ve never read those books so I can say the name without infringing)
Better let AO3 in on that
Not sure why this is downvoted. It's factually correct and is said in what I believe to be a fairly neutral way?
What you describe would almost certainly be considered fair use until point of distribution - it’s non commercial, transformative and has no meaningful impact on the market value of Harry Potter.
Copies for private use are going to be similar, and while I’m not a lawyer it feels like it’d be a hard case to make that work being conducted in private is going to have a meaningful impact on the market for Nancy Drew novels in the next 30 days.