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

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.

Better let AO3 in on that