I have installed several hundred feet of this product over the years and though it is flame
retardant it is not flame proof. Personelly I prefer metal but it is not always practical or cost effective
and since fire dampering requirments in a residence are just about non existant I could see the above scenario happening.
I have a customer who is in process of having a town home built and they ran a 24" piece of this flex duct from his furnace room to the attic to supply air and after numerous code reviews there is no code requiring a damper to stop a fire from going up it.to the attic as he is worried about so out of his own pocket he installing one as a precaution.

I have often wondered how many residential fires could have had there damages limited by effective use of thermal accuated dampers but it apears the construction industry has no intrest in it due to costs. It is hard enough getting them to use it in commercail structures as it is.