This is less of an issue (if any) in Lemmy, but at least in Mastodonland, the fact that you can boost/retweet stuff to other people does mean that a defederated instance can see your content boosted through a “neutral” instance. There were several instances like “dot technology” who went beyond defederating not only the crap instances, but instances that refused to defederate from said crap instances to lessen this problem over there, but with Lemmy not having boosting this isn’t really a problem over here.