Kind of. My observed behavior of Lemmy, combined with comments from some developers (I haven’t read the code):
Post goes up on community hosted on instance A, Message goes out to B and C: “here’s a new post”
User x@B comments on post. Message goes from B to A saying “here’s a new comment”. A adds the comment, then sends a message to C “here’s a new comment”
User y@C upvotes the comment. Message goes from C to A, then A sends a message to C.
Each of those messages are confirmed by the recipient, and there are timed retries. However, there have been plenty of cases where one of those messages get lost, and the communities get out of sync. As I understand it, the message traffic is only changes. They don’t talk to each other to see what the current state of the content is. So whenever a sync break happens, it is permanent. New content/changes are fine, but stuff that gets lost in transit is lost for good.