They don't need to if you're taking a several hundred tonne projectile and throwing at a skyscraper at at least 100 mph. Aside from the thermal energy you've got a considerable kinetic energy in there too which will raise the temperature a great deal. You're also forgetting that the fires would be burning in confined spaces which would cause further temperature increases.
I will have to disagree with that. Fires in confined spaces would be oxygen starved. They melt steel in blast furnaces. If anything projectiles moving at 100 mph would cool because of the wind. I don't think i've ever heard of buildings melting when they collapse.