I'm not so sure about that. My grandmother, who certainly had not seen an iPad or iPhone in action before (she's 85, doesn't speak English and doesn't watch TV), figured it out within one minute of using my iPad for the first time yesterday.
If you think about the pinching movement, the distance between your thumb and index finger correlates to the size of the object. Increase that distance and the object grows in size (i.e. you zoom in). And vice versa. This is what I'd like to call "hyper-intuitive" and is the kind of thing people can figure out without being trained on it.