What is the chance of getting hit by further broken pieces of that satellite and other satellites?
When calculating risk, you have to take into account how many are there and what is the chance that any will be hit. Then you have to calculate what's the chance this will happen again, etc - and only then you can calculate the risk to your own satellite.
It's true that the chance of getting hit by one broken satellite is small. But that assumes there are exactly 2 things on the orbit.