It’s nothing to do with “strong opinions” on snap and how it works fine for other things. It works fine for other things on Ubuntu. Just like Flatpack works fine for things on Fedora. The problem Fox is trying to solve is how heterogenous people’s systems are. Docker homogenizes them.
Personally, I don’t think Docker is the solution, either, but that’s because I think this is the wrong kind of application to run under it. They’re cattle, not pets. TT-RSS is a pet. You run it long term. It’s not really designed to be blown away every 24 hours unless you keep your data in something that’s not containerized (by “data” I mean “the database”). At least in my view (obviously there’s reasonable disagreement).
Going “Docker only” as the supported method certainly is a nice “fire and forget” solution to trying to juggle the questions of every random “Linux enthusiast” who installs TT-RSS and then posts about their problems here. My preference would be “nothing bespoke.” For example, “run this on a supported mainstream Linux release, fully managed by you (i.e.; not shared hosting), running distribution provided packages, or you’re on your own.” That should cover all of the snowflakes without going full Docker.
Me, I plan on going full snowflake. I want to get out of the Linux game entirely and move to FreeBSD (at least for my own stuff. I still manage a swarm of Linux machines at work).