The heater it’s self was 1000 degrees, They do get extremely hot at the source it’s self before blowing it through the house. But it was probably only 130 or so in the whole house it’s self.
Well if the thermostat never tells the heater to stop heating, gas will continue to burn, and the heat exchanger metal will continue to get hotter... its pretty safe to say that the heat would continue to heat up the metal housing of the furnace at and above the actual burners and that that metal would eventually glow red hot... looks like this thing was running non stop for days, really not hard to imagine that it heated itself up that high.
Now, that’s the temperature of the furnace itself, not the temperature of the house... however if the furnace was running non stop It would not take too long even in cold weather situation for the entire house to heat up to intolerable temperatures....
However, humans tend to realize this and will open a window or wake up and go outside or something not just sit in unbearable hot temps until they get cooked to death.
First instinct would assume that they had a carbon monoxide leak and they died in their sleep and then the bodies got baked, but they claim that was not the issue... so it is curious that they didn’t leave the home, or open a window, or something rather than just sit in heat and die.