We're not talking about remembering whole Wikipedia articles though, and I'm guessing nobody puts entire encyclopedia articles in their Zettelkasten. Granted, we're not made to remember TBs of data, but the information I envision to find in somebody's Zettelkasten (well, mine, if I had one) is far more amenable to be remembered via SRS than TBs of random data. That was my conditional "provided we can retain the information": in my case, it'd be just as costly to enter it in my Zettelkasten as it'd be to remember it, so why not remember it instead?
I guess I'd need to see somebody either solely using a Zettelkasten, or Zettelkasten and SRS, and see what's in the Zettelkasten that I couldn't put in an SRS just as conveniently. It must also be content-specific; maybe I'd see a benefit if I were a sociologist like Niklas Luhmann was, but I'm not.
Another thing that changed since Luhmann's time is just how cheap it has become to retrieve most information. His Zettelkasten probably allowed him to quickly fetch information he remembered vaguely, faster than it'd be to open his books and see where it is. This benefit is mostly gone now that there are even faster options.