Actually, the architect that designed the buildings designed them them to withstand several direct impacts. So, in the final analysis, this statement couldn't be more false. The structure wasn't weakened hardly at all. Once the structure was punctured, the load was immediately redistributed. It was a revolutionary and brilliant design.
No one has maintained that the impact alone was enough to bring the towers down. I was merely pointing out that a Boeing 767, impacting such a building at speed, would itself do substantial damage (and they did). The towers were designed to withstand the impact of a Boeing 707, which is significantly smaller.
http://www.pbs.org/wgbh/nova/transcripts/2907_wtc.htmlLESLIE ROBERTSON: We had designed the project for the impact of the largest airplane of its time, the Boeing 707, that is, to take this jet airplane, run it into the building, destroy a lot of structure and still have it stand up.....
NARRATOR: The 767 that flew into the North Tower was larger than a 707 and moving fast. It struck the building between the 93rd and 98th floors, instantly killing scores of people in the plane and tower.
It also created a huge void across six floors on the impact wall. You can see the outline of the wing tip on the upper right. Two-thirds of the supporting columns were completely severed, but the building stood firm.
GENE CORLEY: What happened was that the loads that were being carried by those columns arched across the opening so that the columns adjacent to the hole now started picking up the loads that had been carried by those where the airplane went in.
NARRATOR: Leslie Robertson's radical design seemed to have worked, but there was more devastating damage hidden inside. Although the aluminum aircraft shattered on contact with the exterior wall, the speed and force of the fragments and the intact steel engines severely damaged the columns and stairwells in the core, and jet fuel began saturating the building.