GHz transmission in the 1970s is not special. The cavity magnetron was invented during WWII and used for RADAR purposes and not long after microwave connections were being used for long distance telephone connections.
The magnetron is a good point! But for RADAR applications you just need a big source of emissions that can be pulsed on and off. The magnetron does that well enough. I have never heard of anyone using a magnetron modulated in some scheme to transfer information at any some useful rate. It's definitely possible with gunnplexers. You could certainly do more code with a magnetron, but even that would be difficult given their unstable output frequency.
