What does "support for Fedora" means in this context ?

It supports most OSes rather nicely, check the docs for a long list of config options. It creates a local package store and configures your user's path for it, each tool is managed with a custom plugin that IME works flawlessly and versions are handled better than anything else I've ever used. It's the only way I'll install Golang or NodeJS lately, and I had good luck with it for Java too.

I prefer sdkman.io for the JVM, has most of the things for stuff that runs on it in one place, including Clojure tooling and Quarkus and whatnot.

Ah cool, maybe I'll check it out someday. JVM isn't a huge part of my daily work though so I'm pretty happy having it managed the same way as everything else for now. I like the intuitive simplicity of a .tool-versions file in a repo that supports a huge variety of tools.

Dependencies are in dnf, should just work with the common shells.

Mise also has support

https://mise.jdx.dev/

mise should get precompiled erlang on macos today too

I think Mise is better than ASDF - they have a whole document on why their approach is better than the shims used by ASDF. Both seem fine though.

How does it handle the wx stuff and whatnot that's required for Observer?

Seems to have more project management and make-like capabilities, I can see why someone might find that attractive but it's not for me.