Well, all they need to do is modify the malware to record the audio to a file and transmit it when the radio is next switched on, perhaps interleaving it with normal radio activity.
As a malware detector, this case is useless.
Got a secret meeting? Don't take your bloody phone.
http://www.seattletimes.com/nation-world/even-if-theyre-off-...
In this case it was a Motorola device on an iDEN network, probably using a Freescale baseband. But it is likely that most major vendors were persuaded to include similar capabilities and that network providers cooperate in providing special signaling to control these capabilities.
It pushes the capacity for your adversary to operate into (slightly) slimmer confines, which is a marginal improvement.
Otherwise, your adversary can behave in an unrestrained manner. No?
Only Samsung devices have been shown to give the baseband processor backdoor access to device storage. Afaik.
While it's easy to prevent the phone from recording your conversation (just place it somewhere out of earshot, maybe next to a noise source for the duration of the conversation alone), it's much harder to prevent it from learning where you're going (if you're taking it with you).
This case is not a silver bullet, but it's a useful tool.