Stuart: That's what the manual says. Don't have it in front of me, so I can't give you the exact quote.
But I think I know why they can say that. DMS2 seems to be based on CDMA, which means that each transmitter has is own unique and very distinct fingerprint. Either the RX "hears" that fingerprint and reacts to the information that it carries, or it doesn't hear it and does nothing (probably goes back to failsafe mode is my guess). An FM receiver can't detect lack of signal as easily and will pick up all kinds of noise if there's no signal. Hence the twitching.
(Sorry, but you unleashed the radio geek in me... :-))