OK, if this dust was really from the WTC towers then I would have think it conclusively proves that thermate was used to bring the towers down. The spheres from the dust have almost the exact same composition as one would expect from thermate reactions. This is a big if however. Proving it was indeed dust from the WTC towers collapse might be nearly impossible today.
Proving that this particular sample was from the WTC might be tough, yes, but I am sure that there are official samples all over the place that can be called in for testing.
I wish I still had my baggie full of WTC dust that I gathered when I visited the site in October 01.