I’m a massive fan of GUIDs, too, but you’d have no protection from rogue instances reusing GUIDs of existing posts…

Yeah, everything you said there is correct.

If you want a somewhat more comprehensive definition:
Funkwhale, Lemmy, Kbin (as well as Mastodon, PeerTube, PixelFed etc.) are pieces of software, which can be hosted on a server and which implement a communication protocol for the federation of social media content.

If someone then takes such a piece of software and actually does host it on their server, then that’s called an instance. Generally, they need to buy a domain name to do so, like “open.audio”, “lemmy.world”, “feddit.de” and so on.

You might be interested in Funkwhale instead.
It’s a more mature piece of software, it does federate and they’ve even put quite a bit of effort into podcasts.

They have a flagship instance at open.audio, which only allows Creative Commons content (to avoid copyright issues when federating).
So, as I understand, if your podcast is CC-licensed, they’d be happy to host yours.