At the same time, it might not fit them. Lemmy is a link aggregator, which seems like extra functionality that they don't really need, not when existing forum software will do what they need, while also being more stable/mature.
It's great that they're going back to traditional, self-hosted forums instead of corporate social media for support and discussions, but damn, I don't miss having to manage hundreds of accounts with unique logins for each forum. I understand that they want more control over forum moderation and the Fediverse's "anyone can post there" system makes it troublesome. It would be great if there was more widespread adoption of decentralized, "one login to access everything" systems.
Since I'm now using a password manager I've been having less issue with creating as many accounts as needed.
But I do agree it'd be great to have a single sign on.
But then you have the same centralization issue - and it's even worse, if the central authority has a fit for some reason about you, now you're locked out of many completely unrelated sites.
Federated logins are a thing! The challenge is finding one that's open and privacy-friendly. Unfortunately the widest-used ones come from entities like Google or Facebook with a marked interest in preying on user data. Mozilla used to maintain a federated system (Persona) but they discontinued it. I know Ubuntu offers one for all their services (bug trackers, forums etc.) but not sure if it's open to third party systems. Perhaps there are others worth using.
Alternatively, you can aggregate all your logins in one place across devices and browsers. Firefox Accounts are a very simple method of doing this (presuming you use Firefox everywhere), and you can choose to only sync logins rather than bookmarks, plugins etc. And of course there are other dedicated password managers, with or without online sync, open or closed source, self-hosted or private hosted etc.
I'm sure Jellyfin considered the Fediverse but some projects like the idea of having more control of the community discussions they participate in so having a forum makes sense. I still think a Jellyfin community on Lemmy can thrive with an official forum in place.