That might have been true a few years ago, but they point out that that's not as true anymore. For example, they suggest one practical application of this technique might be to put your own server in a colocation facility, stick a microphone in it and slurp up as many keys as you can. They also were able to get a version of the technique to work 4 meters away, which is far enough that this becomes somewhat different from having direct physical access. They also point out that laser microphones could also be used with this method.