"N-body simulation" doesn't mean what it's normally taken to mean here.

And the colliding gasses models have the huge assumption of random/thermal motion. These satellites are in carefully designed orbits; they aren't going to magically thermalize if left unmonitored for three days.

That's why I mentioned the assumption about the velocity distribution. Sure, the velocities aren't Maxwell-Boltzmann, but that doesn't matter too much for getting a sense of the scale of the issue. The way an astrophysicist thinks (I am one) is that if we make generous assumptions and it turns out to not be a problem, then it definitely isn't a problem. Here they have determined it might be a problem, so further study is warranted. It's also a scientist strategy to publish something slightly wrong to encourage more citations.

Well, sure, they won't be thermally random, but they will be significantly perturbed from their nominal orbits, particularly at the lower orbital altitudes.

Solar flares cause atmospheric upwelling, so drag dramatically increases during a major solar flare. And the scenario envisioned in the paper is basically a Carrington-level event, so this effect would be extreme.

The current "carefully designed orbits" has a starlink sat doing a collision avoidance manuever every 1.8 minutes on average according to their filing for December 1 to May 31 of this year.