http://www.youtube.com/watch?v=BrpxR79TZ6c
 
The action starts at 1:35 with a couple of very respectable Greek-ish sidearm throws.

Looks kinda fake to me, like they just threw the rocks with hand and recorded that.. just saying

I think the end of the video was a kind of sarcasm type humor. And perhaps in the context of the lesson he really explained the that someone was throwing the rocks. I have helped film videos, that out of the context of the lesson, would not make much sense. Maybe it's the same here.
 
At least he's spreading the basic concept of the sling, it's better than nothing. Now all of his students at least know what it is, how effectively it was used, and will probably recognize one if they see it again.

His lesson is good and his form isn't half bad either.  I think the shots at the cans were supposed to be humorous and observed with a grain of salt.

That's one fugly sling. Maybe that's why I can't hit soda cans yet. My sling is too pretty.