I don't see an explanation of how they can test it for real without making space junk.

I gathered that they're interested in creating a "magazine", I know back in the 1980s they considered a "brilliant pebbles" concept of kinetic kill interceptors. They concluded that the pebbles had to float in space in their own and not be stored in orbital "depots" because if you have some platform with N interceptors in it you can kill N interceptors with one missile -- in general (like at sea) the more missiles you gather in one place the more the enemy has a reason to attack the thing.