I think they'll learn more as they go along. The kid could mimic the man when he saw him actually doing it. It was only when the video of someone opening and closing their hand was shown that he didn't get it. Some of us can also dance, but we're otherwise not so coordinated. I don't think all the mirror neurons are shut down in all cases. But, they'll find out more as they look at more brains.
There was another study in CA where they looked at mirror neurons working internally (in regards to ourselves) but not externally (in regards to others) which they think may account for the perceived lack of empathy (in other words we don't give off the same non-verbal signals when we feel empathy and empathy is often an intellectual thing for us rather than quick projection as it is in NTs. So they believe that if we see ourselves in the mirror showing non-verbal signals, our internal mirror neurons will learn them. I'm not so sure about that one. For one thing, I can't read my own eyes and face. Looking at it in a mirror probably wouldn't do much for me. Here's that study. Mirror Neurons