Somebody please correct me if I'm wrong, but I believe due to the very low refractive index of about 1.0003, particles need tremendous energy to produce Cherenkov radiation in atmosphere. So the demon core flashes (while perhaps some negligible part Cherenkov) were probably mostly just from the ionization of the air, and subsequent nitrogen/oxygen fluorescence.

If my awful phone math is right, you'd need about 21 MeV of energy for an electron to produce Cherenkov radiation. I think the processes producing energetic electrons here (Compton scattering, some pair production, photoelectric effect, internal conversion, delta rays, and Bremsstrahlung cascade I believe) should regularly produce energies around 10 MeV at most (from Compton).