But the claim is that you need to learn nix to install apps that aren't in the nix repos, which isn't true.
You can. And then you can even contribute that back upstream and make it available in the repos, but that's not a requirement if what you need to to get an app to run on your system that isn't currently packaged for it.
One of my favorite apps actually wasn't in nixpkgs (don't worry, I fixed that). But I was pretty surprised to learn that it wasn't there.
I'd say what it is, but I'd be doxxing myself.