That's the article I first read about it in. It's great, it really has the main ideas and how to implement it in a paper card system. The "How to Write Smart Notes" book is also great and a short read. It can get a little too pop-pyschology-esque in some parts for me (I just skimmed over those parts), but it is probably the most thorough writeup on Luhmann's system in English.
I had never heard of it, but I found this article about it: https://medium.com/emvi/luhmanns-zettelkasten-a-productivity...