> I found a pdf on some shady private intelligence agencies website that documented a tool to intercept calls and messages as well as locating users of the network. ... Search for "Iridium Interception System reference manual pdf"

This? https://pegasusintelligence.com/docs/iridium-monitoring-syst...

> The screenshots looked like a late 90s, early 2000s windows ui and talked about special radio equipment that the tool interfaces with.

Mid-2000s. A lot of them have dates, and they're all Jan/Feb 2007.