To the best of my knowledge, Mitnick didn't really code at all. There are (let's call them) intrusion specialists whose skillsets don't really involve systems programming, but rather intuition and tenacity, and there are others who write exploits. My understanding is that Mitnick was the former, and was using tools he got from friends and peers.
In the book he spends a lot of time on the social engineering parts of it to be honest. It's been a few years but I remember him mostly bragging about that rather than developing custom exploits.
He also comes from an era of intrusions where systems were so bad you didn't really need to code to get into them. For an alarmingly long time, the most effective tool you could use to pop a network was simply `showmount`.
That time is still today, as people are still the weakest link. A talented scammer can convince people to give them access to their WhatsApp account despite the E2EE, 2FA, and SMS verification codes.
In Mitnik's version, he RTFMs, learned the technical lingo, procedures, and even the names of telco employees.
Yeah I think Mitnik’s abilities were mostly around thinking about doing stuff that no-one had considered that you could do. It’s still a big skill, but nowadays, there’s less stuff that no-one has thought about before.
He didn't really code in the book either... maybe 5% of the book... he did some script kiddie type exploits, some copying of proof of concepts, and some minor modifications (like modifying the "logon" program to save passwords somewhere in cleartext).
75% of the book is spent social engineering over the phone and 20% doing stuff on phone switches and other equipment.
I have met the type on my time on the internet. All it takes is having the guts to push through with what others give you, things they themselves know would get them in legal hell.
Anyone who has studied the later parts of the phone system know that at least a few of his stories are actually bullshit.
It wouldn't be until much later (in the 90s at least, while he was in prison) that the advent of pure digital switching would enable the random reassignment of phone lines like he describes in the story about turning his friend's home phone into a payphone.
The lines were separated and had differences in sender frames just for payphones, plus typical phones weren't too happy when 130VDC was applied to them for very long.
The fact of the matter is that Mitnick went around and shook doorhandles until something opened and occasionally convinced someone to open a door for him her and there, and the fact that the emperor had no clothes was too politically inconvenient for the kinds of companies that Mitnick hit up.
Kevin hasn’t hacked anything at all. He ran with a few other characters who never received anywhere near the amount of attention that Mitnick did. For example, no one ever figured out who “jsz” was.