The thermosphere is a layer of the Earth’s atmosphere from about 95 km to 600 km. It is named ‘thermosphere’ because its temperature increases with altitude. The temperature in the thermosphere can reach 2500°C. However, air density in the thermosphere is very low to the point that heat conduction practically does not occur. Objects in the thermosphere will feel cold.
Flat-Earthers discovered that the temperature in the thermosphere could reach 2500°C. They are happy to ignore the other information which usually accompanies it: the air density up there is also far lower.