IMHO,
For someone to capture computer data that is hooked up to a grid (and not the internet), they would have to place an individual power line monitor for the computer and filter out power surges/spikes to decode in/out data.
I would doubt that it can be used to extract data from the computer except for input/output power fluctuations.
JMHO,
B) xUT
He didn't say they were capturing off the grid, he inferred they were going through a specific AC line into a comp hard drive that wasn't connected to the net. You know, like "Dad, that electric company truck has been outside all week".