Historically the vast majority of zettelkasten didn't have the sort of structure and design of Luhmann's, though with indexing they certainly create a network of notes and excerpts. These examples are just subsets or excerpts of someone's reading of these books and surely anyone else reading any book is going to have a unique set of notes on them. These sets were specifically honed and curated for a particular purpose.
The interesting pattern here is that someone is selling a subset of their work/notes as a set of cards rather than as a book. Doing this allows different sorts of reading and uses than a "traditional" book would.
I'm curious what other sort of experimental things people might come up with? The "novel" Cain's Jawbone, for example, could be considered a "Zettelkasten mystery" or "Zettelkasten puzzle". There's also the subset of cards from Roland Barthes' fichier boîte (French for zettelkasten), which was published posthumously as Mourning Diary.