I probably don't want to know the troofer answer to this, but if you could sneak in and emplace the necessary thermite to do that undisturbed (Not as efficient as high explosives, to be sure, but it would do the job) what is the point of crashing the airplanes? Or is he one of the particularly silly idiots who think they were holograms?
The technical aspects of attempting to cut a steel beam with thermite has been discusses on another board and the general consensus is that it could not be done.
Making horizontal cuts just isn't possible since you would require some type of container to keep a large quantity of thermite in contact with the vertical beam to melt it without melting through the container.
Making vertical cuts in an I beam isn't as simple as it sounds either due to the large volume of thermite required to be kept in contact with the beam.
There was some Troofer group that was going to produce a demonstration to show that it could be done, but here we are a few years later and they have not succeeded.
The troofer just think there's some magical device that the DoD has to do it.