Well, yes, and no.
When such temperatures exist in the absence of oxygen, the super heated fuels form a plasma that flashes as it reaches the oxygen-rich atmosphere, causing a bright, pulsing flame, somewhat like a giant strobe candle. - That definately was not happening there. - It was a slow, smouldering, inhibited fire.
While this is true in an oxygen starved fire which suddenly gets a rush of oxygen (backdraft condition), this fire was vented and free burning around the perimiter where the plane had entered or windows had been knocked out by the people who were jumping.
Just because there was black smoke does not mean the temperatures could not have been above 800 degrees. The adiabatic flame temperature of jet fuel is 3140 degrees F. Even when a pool a jet fuel is burning in open air it will produce large amounts of black smoke. That same jet fuel burning within the WTC was radiating a great deal of the heat to the surrounding structure. It was not a smoldering fire. There were flames visible in many areas of the involved floors.