I think in this case, either the writers didn't really research the area, or the racist hate group likes to exaggerate. I'm going to go with the latter in my game.
(Warning: thought experiment ahead)
In the real-world timeline, Los Alamos had less than 12k registered residents around the same period, and that was only four years after the VITAS plague in the SR timeline, which would have further reduced populations in the area by a quarter or more (NM has notoriously bad health care). Additionally, the area around Los Alamos is (and was) very sparsely populated - one reason it made a good place for labs and test sites. And, we should probably remember that there are a lot of Native Americans in that area of New Mexico (Redondo is just southwest of the Jicarilla Apache Reservation), who would have been removed to a "re-education center" (and thus, why the volcano blew up to begin with).
All things considered, even with no warning, and assuming that the blast effect was able to bury Los Alamos almost entirely with no warning, the death toll would still have been under 10k. Such a sudden and violent eruption would have devastated trees and buildings as far away as Albuquerque and Santa Fe, and denuded an area at least 30 miles in diameter, and would have made the area uninhabitable for years. I can only imagine the background count there if that happened.
It's a fun thought exercise, and gives me a great hook for messing with the A20k people.
