Humanity needs to stop being shit

But that's kinda my point. Anything ejected would only "push off" the asteroid and help nudge it off course, given that they mostly get ejected either in the opposite direction to the spacecraft, or perpendicularly in a roughly even manner (this last one is speculation on my part since I imagine it depends heavily on the angle of impact and possibly more things I don't know, but I don't think it's a wild assumption)

Unless the ejected mass went somehow forward, I don't see how that would steal momentum from the asteroid. Maybe if the ejected mass went perpendicular in a manner was not even in all directions? Kinda stretching it.

Anyway, explodey stuff would only help, not hurt.

I would imagine that the composition would be less of a concern than knowing the exact mass of the asteroid.
A collision in space is as good of a scenario you could get in terms of applying conservation of momentum. Then, knowing that the worst case scenario is a plastic collision (because the probe going through the asteroid completely is very much impossible), you could know with very good certainty the final momentum (assuming we have fairly precise and accurate information on the velocity of both objects). I'm purely speculating as I haven't looked into the mission, but I also doubt the scientist expected a best case scenario of a bounce from the asteroid.