That still only shows replies the server knows about, this article is about making sure all replies to a post are present on all servers.
FEP-1b12 (Group ferderation) kinda already alleviates this, our part of the fediverse doesn't really suffer from the missing replies problem, but it's a solution that doesn't work well with microblog platforms.
I don't know if somethings changed, but it actually doesn't. This lines would need to be
WithContext<SharedInboxActivities>>
for that to be the case, and just to make sure I tested against a local running main and was able to send activities to it without the@context
just fine.