Like those Victorian era drawings of people in posh dresses walking across lakes by having hot-air balloons tied to their bodies..
Even sci-fi games involving space ships and aliens have people using floppy disks, printouts, and faxes.. in years long after those things went out of common use.
Before the iPhone very little sci-fi predicted something like smartphones being so prevalent throughout global society. Today even some of the poorest people in the poorest countries have some form of personal mobile phone.
And even now, the best we can imagine is that people will still be using phones and laptops in 2060.
I do think some sort of AR interface is somewhere in the future but it's almost certainly a long way off for mainstream adoption.
and Larry Niven!
Faster horses!
So much of the things in the book that we think of as familiar didn't even really exist when Johnny Mnemonic was written in 1981, again on a mechanical typewriter.
Modems where expensive but so was a computer, however perhaps people where able to experience the technologies through their workplace or schools investments.
The ideas of Networking/big networks & cracking/hacking/phreaking where not strange in this time. Actually it was probably in the right place to write fiction about. Since most people where not familiar with the terms but had perhaps heard them on the news, advertisements or similar media. Allowing them to reason/dream about networks and the potential of connectivity while the interfaces and had not quite solidified into examples you could easily point at.
For reference the movie Tron came out in 1982. And CBBS was around since late 1970s. And not to forget people in the 1980s where quite comfortable with the phone network. Pagers and early mobile devices (phones/laptops) became available, there was a certain hype/energy about what the "connected" future might hold. A few years later saw the explosion of the BBS scene and technologies like Fidonet where developed and accessible for the computer enthusiast at home.
The point being that while "things in the book that we think of as familiar didn't even really exist" they didn't exist in the capacity we see today. The seeds of the technologies was there even in everyday media channels and it is not strange that a science fiction writer would extrapolate from that.
Asimov has some good stories where people no longer know how to read and write and multiply because everything is done on computers and all interactions with computers and are done by voice.