Based on this I am guessing they didn’t simulate the gravitational effect of any surrounding bodies. If the asteroid core is the only object giving off a gravitational force of course all of the fragments would reform around it. This simulation just feels very incomplete and provides no real actionable results.

My guess is that without rotation ... Why the model doesn't have rotation????? It doesn't sound too complicated to simulate.

My guess is that without rotation, the tidal forces of the other bodies that are far away are small enough to be ignored. In a perfectly even gravitational field, all the parts of the comment has the same acceleration from the other bodies and it can be canceled. (Like the "zero gravity" effect in the ISS.) So only the forces between the fragments of the asteroid are relevant.

Anyway, the Shoemaker–Levy 9 comet suffered some disruption due to gravitational tidal forces because it passed too close to Jupiter, and the fragments formed a line instead of a ball. https://en.wikipedia.org/wiki/Comet_Shoemaker–Levy_9