It’s really unfortunate that GitHub releases don’t leave any room for installation instructions or notes for the individual files. I was looking for a good place to put these notes yesterday, didn’t find one, gave up and figured I’d deal with it later.
The .flatpakref file just contains a link that should install the app from Flathub when you open it with your software manager. (It’s the same file that gets downloaded when you click “Install” on Inform | Flathub) The .flatpak file is a standalone package of the app which you can use if you prefer not to download it from Flathub. You only need one or the other, not both.
The .fc35 indicates the RPM package was built using Fedora 35, it will probably work on later versions and may work on earlier ones. It also may work on other Linux flavours that use RPM packages.
The DEB package was built on Ubuntu 20.04 because that’s the default environment that GitHub provides. Same thing goes here, it will probably work on later versions and may work on earlier ones. It also may work on other Linux flavours that use DEB packages.
It’s correct that I would prefer that people get the app on Flathub if they can! Building all the packages is a hassle and takes time that I’d prefer to spend on actually improving the app. In that light I wonder if in future releases we should instead turn the tables, put a link to Flathub on the official Inform release page, and put all the packages in a still-discoverable but less official-looking place like the ptomato/inform7-ide release page.