That's odd. I've seen documentaries showing that they didn't. But the area that needed to be insulated wasn't the vertical beams (though it would help). It was the horizontal connecting beams that 'unzipped' to cause the buildings destruction. When the horizontal beams became stressed, they detached from the vertical beams holding the building up. This put more stress on the remaining beams on the floor which rapidly failed. While the vertical beams may have been insulated, I very much doubt that the horizontal beams were.
Yeah, I knew that about the horizontal beams. I can swear I saw an asbestos explanation. Or it was someone speculating.
Anyway, asbestos isn't the boogy-man enviros make it out to be when used properly. It's the removal that will get you.