My consideration is that these guys have been trained in coms.

Think of that soldier in VietnamCong prison, hauled before a camera, answering questions. While doing so, sending morse code by blinking his eyes.

So, the effort is rather small.

Compare it to doing arithmetic. Most people can add and substract single numbers quite easily. When it comes to multiplication and division things become more intens, but are quite easy to do when it concerns double digits. Triple and quadruple digits, by heart becomes something of a challenge to most.

So, you learn a skill, and train the hell out of it. The use and effort seem effortless.