I checked on a forum where I have been assigned topics and it does appear that unauthenticated users get all topics, not the assigned ones. It does look like if users can’t see assigned stuff that part of the query is thrown away.
If you don’t want users to know about things that are assigned, it seems improbable that a user would stumble on this query. It’s as though unchecking “allow public to see assignments” means something like “don’t allow public to see whether there are assignments”.