9/11 Experiments: The Great Thermate Debate: https://www.youtube.com/watch?v=5d5iIoCiI8g
require an enormous amount of energy to get hot enough to fail. There generally is not enough concentrated energy in office fires to heat these large pieces of steel to high enough temperatures to cause failure.
It is easy to heat a 1/2" diameter steel rod and then bend it using a large lever arm of about 3 feet. The blacksmith's argument is not germane, because you can't get enough heat energy on the large mass steel structure in a high rise building due to an office fire and Jon's video also shows that with the real mechanics involved it won't fail.
Let's see the Blacksmith bend his anvil.