Yeah that's why it would be good to make deleted threads still show up for all users, they would be unlisted but the comments would all still be there.
It used to be that this would also happen with comments both deleted and removed but this functionality was changed and comments which are deleted or removed no longer error like that.
Lemmy isn't impervious to vote manipulation either unfortunately, I encountered someone in a thread earlier who was getting botted, one downvote for every upvote in lock-step with each other. Good news is that Lemmy admins will squish them if you report sussy voting behavior.