It’s just a matter of math and putting enough garbage into the flight path to damage it enough that it stops.
When you figure that one design for an anti-ballistic missile weapon was basically a giant umbrella frame that was inert because the impact speed would be enough to destroy the warhead.. it makes the term “unstoppable” rather inaccurate.
And nuclear warheads are reentry vehicles that travel MUCH faster than this supposedly “unstoppable” missile.
Bingo! At that speed, any sort of shrapnel will impact it’s capability to hit the target.