Essentially, they are saying that the test was successful but it is still only one data point.

The final aim of this research is to be able to know, in case an actually dangerous asteroid were on a collision trajectory with Earth, what kind of impact would be sufficient to knock it onto a safe course.

So we know the mass, trajectory and velocity of DART and soon we'll know the exact effect the impact had on Dimorphos, but what we don't know is the effect of Dimorphos's characteristics. What if DART had struck another asteroid with, for example, the same mass but denser? What if the asteroid had been composed of a different material?

By carefully analysing the impact site and the asteroid itself with follow-up missions (like HERA) we can try to determine the effect of Dimorphos's characteristics so that in the future we may more easily predict the effect of an impact on any asteroid, provided we have some data about it.