We shouldn't forget that the bus speed of a computer can be the frequency at which it broadcasts. A poorly shielded machine can have its video card output and system bus output captured in say for instance an unmarked van parked near your house. Thus allowing the persons doing the eavesdropping to see your full screen and any data coming across the bus (like what you're typing or clicking on with your mouse).

'They' have been able to do that for a long time

This is very old news if one digs into the history of the Tempest Security specification.