"If I have nothing else to do then I write all day; in the morning from 8:30am to noon. Then I go for a short walk with my dog. Then in the afternoon I work again from 2pm to 4pm. Then it's the dog's turn again. Sometimes I lie down for a quarter of an hour.... And, then I usually write until around 11pm. I'm usually in bed by 11pm where I read a few more things."8
Luhmann his output might be a result of his work ethic and routines. Attributing productivity merely to his zettelkasten is misleading. Also Chris Aldrich on The Cargo Cult of Zettelkasten https://chrisaldrich.wordpress.com/2023/02/03/a-note-on-the-cargo-cult-of-zettelkasten/