I also am a little skeptical about the ability to move the market with $10mm. However, he said $10mm in capital, not "$10mm." That could mean $100mm or $1bil in stock depending on leverage. In fact, I think he specifically mentioned futures. What people seem to be overlooking is that if a hedge fund manager invests additional capital to drive the market lower to protect his short position, he risks digging an even deeper hole for himself if the market does not respond as he desires.
I have a hard time separating the simple activity of selling short because you think the market is going down, and selling short with the intent of pushing the market lower.