Usual 'weak' evidence 'documentary'
I agree (as an ex Metallurgist) that temperature (heat is a different thing) can effect the strength / ductility / hardness etc of steel. Depends on the temperature and time and removal of the heat source ( quenching) together with the original materials consistency. And that is a simple explanation of the variables.
I haven't watched the program, so maybe I am being over critical and dismissive. Did they find records of any of the above variables....appears to me they can't even validate the occurrence of the fire in the first instance.
Or maybe after 15 years of watching the World Trade Centre fall down they have decided the viewing public deserve greater enlightenment.
From David in Thailand, sat on a beach, watching people enjoy themselves making their own entertainment.