A little more context
DART struck Dimorphos in September 2022; its larger companion is Didymos. The mission was a controlled test of one technique, not evidence that every potential asteroid threat can now be handled the same way.
The spacecraft’s impact was only part of the effect. Material thrown off the asteroid also carried momentum, helping to change its motion. Observations after the collision let researchers refine the orbital measurement.
A future deflection would depend on the target’s properties and the warning time available. DART gives mission planners real measurements to learn from; it does not remove those uncertainties.