Yep, we shouldn't speculate too much, still another my attempt
My original thought was: there was apparently too little of carbon in the WTC construction steel(s) to explain microspheres in Bentham paper as cementite spheroids formed from gray layers, but couldn't be some cementite formed during burning of paint by reaction with its thermally degraded carbon-based polymer binder?
Probably yes, since the formation of cementite from hematite is well-known, when heating e.g. with charcoal
(which is what is basically formed from any paint polymer binder above ca 450 degrees C, until it is not burned out by further oxidations at even higher temperatures). Also, steel itself "consumes" some carbon e.g. from charcoal, with the formation of cementite during "carburization".
But, these are again just speculations based on very quick literature search, microspheres can be indeed something else than cementite