If we were talking about a projectile fired at relatively close range on the surface of the Earth I would agree with you. But we are talking about a very small object very far away with already only a 1% chance of impact. Buckshotting the object with a random explosion is unlikely to change the dynamics of system to point where you say with high confidence the chance of impact is much higher. Some fragments might hit but it's very difficult to predict.
As long as some part of the asteroid keeps the old trajectory, the chance can only increase because you’re adding particles with slightly different trajectories.
