I’m assuming they mean it isn’t a two way block. Blocked users can still interact with your posts/comments, you just can’t see them. I personally think that should be how it works, but I’ve seen a lot of arguments for the Reddit-style blocking where they can’t interact with you anymore.

User level community and instance blocks will stop you from seeing posts from those places, but it does not block their users or their comments, so you'd still be able to see them around in non-blocked communities.