Muppets in a hot air balloon

I have recently read that during the opening sequence, there were two helicopters flying around the hot air balloon. One was for a camera to film the scene. The other was for the performers to radio-controll the mouths of the characters in.

But I think it's strange that this was needed. For the longest time, I thought that a real hot air balloon was only used for wide shots, where you can hardly see the Muppets, much less what they are doing. I thought that close-ups were filmed in a studio, with the background resembling a sky, and the performers being under the bottom of the hot air balloon. After all, in all shots where they can clearly bee seen moving, we can't see any parts of the sky that could be seen under the view of the hot air balloon. But then I realised that the Muppets hardly move their arms in the hot air balloon.