But my impression of it on Swedish TV was completely different from yours.
Firstly when interviewing the whistlers you could tell that it wasn't a language. It was just a set of sounds they had taught each other for specific situations.
When they were standing 20 metres from each other and tried to say things like "take off your hat" or "pick up the shovel" they had to give visual cues after trying the whistle a few times and failing to get the message across.
So sure, they whistle across the valley, but just to get a very limited message across that concerns them and their daily work perhaps. Not to have conversations.
The swedish TV host tried to learn the language and also went to some bar to order. Maybe they do that as a standing gag there, with the ordering. But he didn't get it right until he played some school children whistling on his cellphone.