What many people ignore from Ahrens description of Luhmann's Zettelkasten, is that it isn't an indexing system to find pre-existing knowledge/texts. It is the place where he stored his own insight, his own summaries, his own formulation of ideas he found somewhere. The book quotes Luhmann even before the table of contents with "One cannot think without writing." Strictly speaking, this is obviously not true. Metaphorically speaking it is something that many proficient authors have concluded as well. They need to write in order to make progress. And then Luhmann used this one space of notes, the Zettelkasten to work on all his publications. This allowed him to find shared ideas between different projects of him.

I've got the impression that many tried Zettelkasten as the one true note taking system just dumped stuff they found in blog articles into pages in their storage system for later retrieval. Nothing is wrong about that, but that is not Luhmanns idea of Zettelkasten.