In the good old days when MTV actually played music and people were using these huge computer monitors with these ray tubes ( dont know how to translate this propperly ) one of the coolest ways to spy on someone was to receive the radio waves produced by those tubes and then use this information to duplicate the image on another monitor in a room next to the actual monitor, so you can see what someone was doing on his computer without ever touching it, installing software, or in some other way gain access to the machine.
If you knew a little bit about electronics this was actually not so hard to do.
I guess what is described in this article is simillar to this, it reminds me of a paper i read where researches listened to the sound that a CPU makes while de/encrypting data and from that sound calculating the key ( = passphrase) that was used. This even worked if they recorded the sound with a cellphone. This sounds crazy but this totally works, in fact this audio stuff is becoming a huge problem in IT Security.
I am a Programmer and i know a quite a bit about this IT Security stuff, if the NSA could/would do this it wouldn't suprise me.
EDIT: I just found the paper where i first read this, if you really want to get into this ( highly recommended ) read it:
http://www.tau.ac.il/~tromer/papers/acoustic-20131218.pdf