Seems like an interesting effort. A developer is building an alternative Java-based backend to Lemmy’s Rust-based one, with the goal of building in a handful of different features. The dev is looking at using this compatibility to migrate their instance over to the new platform, while allowing the community to use their apps of choice.
C# is regularly under-represented in OSS, in part because for most of it’s existence, the primary implementation (.NET Framework) was not open source or cross platform. It is also very popular in fields where open source is not the norm (game development, bespoke backend infrastructure, embedded apps).