Haven't seen the movie Deliverance, but the clip you shared here is interesting (I thought you didn't like this kind of music?) I like the development of the scene, from a pair of repeated notes to the whole improvisation and the whole song and joy and them themselves and those who listened. You see what music can do :)

Have you seen the movie at all?

No, actually dont know ;D but I clearly remember seeing this opening in the house of a friend of mine. She invited a few of us to see the movie but I really can not recall anything about the movie later. I suppose we changed the activity we were focused on.
Now, the music from it, as I checked the other link, the soundtrack, but was skipping parts, there is Ligeti, the composer I mentioned :)