Fire is combustion of gas. Nuclear fireball is light and heat from plasma created by the tremendous energy being released. Like a phosphorus or neon light when gas in the atmosphere is heated ( has energy added) it lights up. Also there is other light from the radiation itself reacting with the gas but I think most of the light is produced by the tremendous heat. Just like a heat shield glows.
There isn’t combustion like traditional explosives but the light and heat released was strong enough to burn most things where they stood.