From what I understand the default behavior of NSFW communities is that remote posts are effectively treated as hidden and don’t show up unless you’re subscribed or directly access the community. However, Lemmynsfw explicitly altered this behavior such that their posts go into main feeds and were treated as normal (though marked as nsfw).

I was never subscribed to any lemmynsfw community but would routinely see their posts scrolling through top day, so I blocked the instance and now I don’t. Very useful feature.