Not only do I believe that he has used encoded messages I believe he'd keep doing it as long as we were dumb enough to let him.
Of what use would be a message that would depend on american forces finding a tape in a shack in afghanistan, running it all the way back to the states and broadcasting whole on CNN. what would it say "strike now"? so if the tape had never been found no strike would occur? I don't know if you're aware of this, but usually, when communication needs to occur, the timing of the message is important. Like say, the code to pull out of vietnam (bing crosby's "white christmas") wouldn't have helped so much if it depended on the VC broadcasting it. And if you're worried about sleeper cells co-ordinating attacks, then shortwave radio could fill in that void.
I think its just useful, mystifying paranoia.