I guess I’m kind of asking / supposing, but I wouldn’t be surprised at all if lemmy / activitypub doesn’t really work that way.
As in, posts to feddit.uk are stored on feddit.uk. If you read a thread from sh.itjust.works/c/whatever@feddit.uk then sh.itjust.works server will pull it down from feddit.uk and show you the post. It may even cache a copy of the post to show it to another user, but that’s not the same as backing up the post.
As with any law, there will undoubtedly be cases in which it is difficult to discern whether or not a law has been broken, but courts decide on innocence or guilt in such cases every day. A jury would be asked to decide whether a a reasonable third party is likely to conclude on the balance of probabilities that the image depicts a person who is under 18.
Whether or not the depicted person is real or imagined is not relevant in many / most jurisdictions.