I dont think this is quite right - lemmy.world can't listen to Beehaw either, it just still has the synced copies of the Beehaw communities loaded on it. So lemmy.world users can still see posts and comments from before the defederation and can even comment themselves but nothing gets synced between the two instances. Effectively there are separate instances of those communities now, one on Beehaw and one on lemmy.world. To be honest this way of managing defederation feels a bit misleading as it gives the impression there is still a connection, IMO when you browse a community that originated on a defederated instance it should be something really obvious and upfront.