The Rand analysis assumes :
1. The bigger rocks are not of strategic interest They limit the consideration of destruction to roughly nuclear equivalent. I think that is an incorrect assumption. Rapidly advancing technological capability could require more destructive capability to penetrate advanced defences
2. It does not consider significant advances in space travel capability. Thus it only considers the 900 or so asteroids that make very close passes to the earth fairly regularly.
3. It does not take the longer view where technology changes substantially.
Molecular nanotechnology and some other potential technologies could provide far superior travel capabilities which would alter the analysis of whether it is worth it to develop those weapons.
MNT and other new tech will up the ante from the current nuclear status quo. Thus more unstoppable destructive capability will be considered. If we go into a period where there can be surprising and rapid advances in technological capability, old weapons arsenals could be rendered obsolete or could be overrun and taken by an enemy with new technology. Developing space rocks as weapons would be a far more secure long term strategic deterrent. They would be like submarines with a nuclear arsenal that would launch a reprisal year or years later.
I have written several times about using space rocks as kinetic weapons. Most recently noting it only takes advanced near term space travel capability. We would also likely have quite good metamaterials which could be used to make the rocks that you have picked invisible on several wavelengths