Yes, it is, because the explanations offered so far have been limited to "This must have been done by a thermitic reaction". Nobody has outlined a mechanism by which a thermite reaction could have produced this exact morphology of material. So, no, we don't know of any way thermite could have produced this effect, and nobody has come up with an explanation.
As for the experiment in which Cole claims to have proved that the sulphur could not have originated from the drywall, there are several fatal errors in his work. He made no attempt to reproduce, or even measure, the temperature seen by the steel in the rubble pile. He made no attempt to reproduce the duration of the heating of the steel; his total heating time was one or two orders of magnitude too low. And finally, he made no attempt to determine whether any sulphidation of steel had taken place; he simply judged by eye that there was no sulphur in the steel.
So, he didn't set up the conditions correctly and he didn't measure the results. It's a bit difficult to draw any useful conclusions there, don't you think?
Dave