

Sorry, but this will be a bit too technical…
The thing is that Lemmy (at least, others probably do the same) don’t treat the Linked Data as the canonical representation, they work by translating every message with an as:Activity to their own internal representation in the database (with separate tables for Post
s, Comment
s and PrivateMessage
s).
This means that all it takes for a Lemmy instance to treat a post as “new” comment is to produce an “as:Announce” attributed to the “follower” community, and then all instances will process it as a new post/comment/vote.
They are still separate communities. Users following only A will not see the posts from C. Users following both A and C will everything.