Oystein,
The information I got from many chemists and others convinces me you are probably right, but because of my lack of training as a chemist I am giving every benefit of the doubt. Kevin Ryan insists it's a good experiment with only a few things that could be improved upon, and he gave me reasons.
What's easy about this is that none of our opinions will much matter if the dust is submitted to RJ Lee. A good, standard lab test from a dust laboratory will yield an answer. If it's positive, then someone would have to get another dust sample with a strong chain of custody.
Honestly, I doubt the test will ever happen, and if it does, it will test negative for thermites. But again, my opinion is irrelevant. Either they test it right or they don't. I await a reply from them. Like the Grateful Dead said, Your cards ain't worth a dime if you don't lay em down.