Also, the formalism is the standard way astrophysicists understand collisions in gases or galaxies, and it works surprisingly well, especially when there are large numbers of "particles". There may be a few assumptions about the velocity distribution, but usually those are mild and only affect the results by less than an order of magnitude.
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.
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.
https://www.scribd.com/document/883045105/SpaceX-Gen1-Gen2-S...
It also notes that the collision odds on which SpaceX triggers such maneuvers is 333 times more conservative than the industry standard. Were that not the case (and they were just using the standard criterion) one might naively assume that they would only be doing a maneuver every ten hours or so. But collision probabilities are not linear, they follow a power law distribution so in actuality they would only be doing such maneuvers every few days.
It is disingenuous to the point of dishonesty to use SpaceX's abundance of caution (or possibly braggadocios operational flex) as evidence that the risk is greater than it actually is.