It's not about pessimism it's about the difficulty of simulation, and lack of detailed knowledge.

And no, this is not a simple inelastic collision problem, it's not simply a matter of the final momentum of the asteroid being its starting momentum plus the probe's momentum, if it were we wouldn't have sent the probe. The asteroid is a rubble pile, which means that the impact ejected a huge plume of debris out of a crater. Because it sends a debris plume backwards (and that momentum needs to be balanced) you get greater than 1:1 momentum transfer, essentially turning the crater into a rocket engine powered by the probe's kinetic energy. The details of that plume depend greatly on the compositional and structure details of the asteroid, something we have very little firm data on up until now. We have literally fewer than five data points to go on for this sort of thing. So folks put together some simulations with variations according to the knowledge we have. As it turned out in this one instance the result was on the high end of all of the simulations, well above the average.

One thing worth pointing out here is that this is still just one data point. It may be that dimorphos is actually an outlier in terms of its compositional structure. Or it may be that the particular spot we hit was unusual. The average could be lower or higher than what we achieved with this specific instance. That's why we need a lot more studies like this one to collect enough data to be actually of practical usefulness.