“Identifiers” would be more like addresses. A computer metaphor: if you have files, you can think of the path to the file as its identifier. Luhmann used an alphanumeric code (which I think is obsolete on the computer) but you can go with date-time stamps like 202002270807 for 2020-02-27 08:07 or similar. Or you use the title of the note. But then you cannot change the title anymore without breaking links, which makes it brittle.
Linking essentially works exactly as you described: leave the address in other notes in your archive.
Re: Luhmann’s productivity, see the list of published books (! not counting articles !): https://en.wikipedia.org/wiki/Niklas_Luhmann—he was a prolific producer of texts.
You’re right, all this can be done on a computer just fine. And you even get full-text search and such quality of life improvements.
The “key” to his productivity was not just doing all the steps you mention mindlessly for 30 years and voila, you have tons of books. Have you ever contributed to a wiki, e.g. at work? When you know 10 things but none of them are on the wiki, you create pages for them, add cross links, then find opportunities to add more details, explain the connections a bit—you begin to work with the material, massage the existing pages, come up with new stuff in the process. Meanwhile, the rest of your life takes place, so you read, learn and hear about things outside of this hypothetical wiki, which may bring up more ideas to add to the existing knowledge base.
That’s how working with a Zettelkasten is. The “key” is to create structures over time that can solidify knowledge and aid in attaching new ideas, discovering holes in your knowledge, and such things. To find out what Luhmann really did in practice, you may want to read his short “Communicating with Slip Boxes” article: http://luhmann.surge.sh/