The smart guy was correct in his assessment that more work would be done from within. But the only reason it worked to the Earth's advantage was that the asteroid cleanly broke into two pieces just in time to miss.

Realistically, his plan possibly would have been a worse scenario, splintering the body into many pieces that would impact in several areas. But given the setup, there wouldn't really be a good way out. It's time that's the big factor, if we don't have a lot of that, even a small rock would be a problem.