That sentence has perhaps not been edited in a while. .NET has been multi-platform for years. (edit: my bad, I didn't see that you'd linked to the .NET Framework, which is the Windows-only legacy runtime.)

There are several GUI libraries that are still Windows-only (e.g. Winform, WPF, etc.), but the latest one MAUI targets MacOS as well as Windows desktops, in addition to iOS and Android.

Just glancing at the code on GitHub, Jellyfin is primarily an ASP.NET web server targeting .NET 6. The download <https://jellyfin.org/downloads/> page shows a wealth of platforms, of which Windows is only one.

The original statement suggested the Jellyfin was somehow weaker on other platforms than Windows when there's nothing to suggest that there isn't parity on all supported platforms.

The .NET team has put a lot of effort into shedding its Windows-only past. I'm honestly not sure how FOSS the platform is, but at least some of it is open-source.