The authors of the paper state that "the numbers, sizes, and shapes of the boulders imaged using HST are consistent with an origin as preexisting objects dislodged from about 2% of the surface of Didymos, in a circular patch 50 m in diameter or larger."
The penultimate image recorded from the DART impactor shows boulders on Dimorphos in a field 30 m wide. The yellow circle marks the nominal DART impact location. Some of these boulders are now free-flyers!