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.
I think the developer of PieFed is mistaken because the microblogging projects also use shared inbox a lot. My understanding is that for certain classes of posts, they actually just use it over a user’s individual inbox and the remote server is responsible for delivering it from the shared inbox to the user’s timeline.