Mitnick's use of the sequence number spoofing exploit was a super big deal at the time; it's half of the centerpiece of his weird dramatic struggle with Tsutomu Shimomura, whose server he broke into with that exploit (the other half was Shimomura helping use radio triangulation to find him).

Mitnick didn't write any of this tooling --- presumably someone in jsz's circle did --- but it also wasn't super easy to use; spoofing tools of that vintage were kind of a nightmare to set up.