The only connection between the Theremin spy tool and the Theremin musical instrument is that they were both associated with people named "Theremin." I'd have to check very old references to verify that it's the same Theremin person in both cases, although that's not really critical information. The date for the spy device, and what I recall about the appearance of the musical device suggest that it probably was the same person.
The operating principles of the two devices are significantly different. I suspect you may be too young to have built one of the musical ones when they were first a fad.
The spy tool mentioned in the Wiki article is only an example of a first use of absorbed and retransmitted rf power, and is almost extraneous to the discussion that follows.
The link (http://en.wikipedia.org/wiki/Radio-frequency_identification) is a good one, and does provide some information, including a few comments on shielding of RFID devices, but may be most notable as one of the longest articles at Wiki (26 full screens on my monitor). You and I probably will be the only ones here to read the whole thing(?).