The flame was confined, and the windows at that height were the perfect bellows.... I'd GUESS.
The steel didn't need to melt, just weaken.
I don't know how hot jet fuel burns in open air but I'd guess hotter than coal, which can melt steel... I think around 1500 F.
Thermite is aluminum and rust. It works by acting as an accelerant. It has an extremely hot flash point.
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