Hi Nicole,
If the posts from the "standhouders" category is not viewable by guests, try viewing the link as a guest(non-logged in user). You can see that the category's post/s are not displayed in the feeds. From your backend, there is a post under that category that starts from "Kies...". If you view the rss feed link that you provided as a guest, you would not be able to view that post in the list, which means the privacy of the post in rss feeds format will inherit from the post itself. Hope this clarifies your issue.