The fact that someone can upload illegal content to a lemmy server doesn’t change whether or not it is associated with a post. The two are mutually exclusive issues:
moderation of user submitted content
moderation of abuse of hosting functionality (illegal or otherwise)
Both are real issues that need to be addressed, obviously, but it’s simply not the case that a server admin’s only visibility into the content hosted on their server is only that which a user associates with a post. If you know any admins like that, do them a favor and let them know they have no business running a lemmy server.