How to recover storage from "Videos" pseudo app?
So a system restore (I actually upgraded to iOS 13) fixed the problem. I had been trying to reduce the size of the backup before upgrading, but perhaps it would not have affected the backup size.

This article gives background info on why there might be "zombie" storage usage even after deleting the files: How to Delete Documents &amp; Data on iPhone or iPad. (TDLR: "caches, app data, preferences, login details, and assorted other app-specific information")