The Killer Fires Theory is Pure Fantasy

The simple facts of temperatures:

•   1535ºC (2795ºF) - melting point of iron
• ~1510ºC (2750ºF) - melting point of typical structural steel
•   ~825ºC (1517ºF) - maximum gas temperature increase imparted by a hydrocarbon flame burning in the atmosphere without pressurization or pre-heating (premixed fuel and air - blue flame)

Diffuse flames burn far cooler.
Oxygen-starved diffuse flames are cooler yet.

The fires in the towers were diffuse -- well below 800ºC.
Their dark smoke showed they were oxygen-starved -- particularly in the South Tower.