Sadly, I cannot remember the method they stole it from. I do remember it was created by a man working for Hewlett-Packard in Britain who published a book around 1993.
TCP/IP Illustrated by W. Richard Stevens has sequence diagrams, although that book dates to 1994 which is right around the time UML was originally being developed. So, it's not a strong data point.
I'm positive you're right that sequence diagrams existed independently of / prior to UML though.
Sadly, I cannot remember the method they stole it from. I do remember it was created by a man working for Hewlett-Packard in Britain who published a book around 1993.
Does anyone remember?