say there are three instances, A, B and C. if A is not federated with B, users from those two instances can’t interact (right ?) but what if both are federated with C ? would both indirectly be able to interact ? if there is a post on instance C and a user from B comments on it, would that comment be visible on all three instances ?
I think it depends. Not entirely sure though. Now given the context of Lemmy right, you can certainly post to instance C from both A and B. Even if your instance (either A or B) will not fetch messages from each other (due to not federating) you can always go see the messages through instance C (not logged in). That probably works for all defederation scenarios but that is also a very loosely defined interaction as both parties will have to do the same.
A diplomatic answer I know.