IIRC, the WTC towers employed an innovative construction technique. I do not recall the details, but it relied on steel cables in tension, something like a suspension bridge. Once the heat caused the cables to go into yeild (get soft), the whole thing collapsed like a house of cards. It had a considerable amount of redundancy, but more support was lost on the side of building towards the entrance wound, unbalancing the contraption and reducing the margin.
I don’t know the details on the WTC, but you’re talking about post tension cable construction. This is used to make concrete slabs capable of taking tension stress. Hitting the softening point in a cable in a case like that would surely make the slabs fail. Also the framework would have been steel girders supporting those slabs, and those beams would also have buckled after hitting the softening point.
I don’t remember the details completely (not a Civil Engineer anyway), but there is a pretty small maximum height you can attain when you try to build a building with just masonry and not steel. Steel had it’s strength based on a couple of things- work hardening and precipitation hardening. Once you get to the softening point of the steel, both of these factors crash, and you end up with really soft metal.