This is an animal living in the wild who did not have contact with humans. The article mentions that he may have learned it from other orangutans, but says nothing about humans.

Yeah, i read that but i don't think it tells the whole story. In Borneo they live in close quarters with Humans and often observe people (i.e. curiosity) from the forest's edge and may have picked up something via this way. They are very clever primates and hence could have picked up this behaviour in any number of ways.

PS: Also checkout the Orangutan Jungle School video i link here - https://news.ycombinator.com/item?id=40244125