Given hot metal distributed by the air in the collapses probably landed on plastic and that plastic could smolder for hours your bare assertion is nonsensical in the least.
If thermite dispersed cooling in air can light a fire, so can hot carbons, or hot metals, your desperately grasping at a straw man argument.
Just like saying the steel can not rapidly fail, an engineer, who does not understand weld and bolt connection failure, impossible more impossible than a building falling at free fall.
Also gypsum in dust form is light and grandular an inch of gypsum would only induce smoldering not prevent fire spread, any desturbance of the layer, would disperse it allowing for flame propagation.
Any fuel oil leaking on too a smoldering fire under gypsum would soak into the gypsum and ignite it.
PS. Just for fun yesterday I tested your gypsum Idea I placed a piece of heated steel 380C on paper covered by one inch of wet gypsum powder, the Owensboro Messenger, began smoldering in five minutes, although no flame was produced until the light wind disturbed the gypsum layer exposing the paper to air.
These Ideas are easily experimentally tested Tony and you fail miserably.