Also just thought I'd get in on the hilarious debate here
I don't doubt that Emby has had user-facing improvements since it went closed source, but as a developer I see it as basically the same as Jellyfin still. In the initial development of Finamp, I used Emby's documentation to implement core features such as auth, library navigation, and serialising/deserialising data. This all... just worked in Jellyfin, and now that Jellyfin has better developer documentation, I can see that those bits basically haven't changed. Even their audio transcoding stuff is the same as far as I can see. I've been meaning to see if Finamp would work on an Emby server, and I'd be surprised if it didn't.
And, as everyone else has said, it's totally fine for Jellyfin to fork off Emby. If I made Finamp closed source tomorrow (which I doubt I could even do without all of the contributors agreeing), everyone has the right to take that final code and continue the app as free software. I made that decision when I picked the license for Finamp, as did Emby.