Since the rhythms of village life tend to be fairly predictable, the speakers of this language are obviously being aided by their mutual knowledge of what is likely to be said. Though they can exchange a surprising amount of specific information this way, it would probably not allow them to carry on a complicated political discussion. Whistling is by no means the only way to communicate a melody. These are also relying on the importance of tones in the speakers' languages. The curiosity of whistled speech aside, all this reminds us of at least one significant thing about language.
Tone languages put heavy reliance on the tone that has to go with each and every word.
And of course when we can see each other, gestures and facial expressions. Even the simplest speech act is making use of all these complicated channels at the same time.