Zettelkasten is the original name of the concept of networked note-taking. It even worked on paper. Roam took the concept close to users with a polished UI. Some history: https://fortelabs.co/blog/how-to-take-smart-notes/
No, it's not. Zettelkasten is just the german word for "Box of paper slips", a common tool for certain people. There are many famous Paperboxes from artists and scientist over the centuries, but the ojne responsable for this new zettelkasten-cargo-cult would be Luhmanns Zettelkasten.
Niklas Luhmann was a sociologist who had the habbit to gather notes to an excessiv level on paper slips and collect those in boxes. He was famous because he used common organizing-methods of scientist for notes, instead of just quotes, books and articles.
Some years ago serious research started about his notes, which are now all scaned and online available, and since then some kind of irrational cargo-cult around him has started.