What does "cable" mean?

The word "cable" has its origins in the days of the telegraph. Messages sent internationally via undersea cables were known as "cablegrams" or "cables", for short. Another interesting point to note is a cable (the means of transmission) is insulated and protected from external elements, distinguishing it from an ordinary wire, which is just bare metal. In the early twentieth century, governments and agencies communicated via cablegram and the name has since stuck. Another reason I would suggest for this enduring usage, even with the advent of modern telecommunications, is the security and encryption involved in relaying these messages. The messages are sent via a secure link and chances are that the signal even travels through an undersea fiber optic cable before reaching its destination!

Back in the day, "cable" was used to describe communications sent abroad. In the case of Guardian, it seems to refer to news from overseas.

"Cable" in this context, means "cablegram." It refers to a report sent via a cable, not to the cable itself.