You should keep in mind that FEP-400e is nothing more than documentation of how group federation works in one specific project (Smithereen). According to this comment, there was no consensus achieved with any other project when the FEP was finalized, and it seems that no other project has implemented the FEP so far. You can also see for yourself in the socialhub thread that there were many open discussions both before and after finalization was requested, which were never resolved.
On the other hand, Im curious what you think about Lemmys implementation of group federation. Its quite simple, persons send activities with the group in cc, which then announces it to all followers. This means that its already compatible with many other projects (including Mastodon), unlike FEP-400e which would probably require changes in all involved projects to support it. Its also worth noting that Lemmy is the largest Activitypub project with group support, and the implementation is already proven to work in production without problems.