Agreed to all. I will talk to the inspector if he ever calls me back.Only if things are different in PA, otherwise no, that's not generally true, you can use NM in fire-rated assemblies in residential work. You'll need to be careful about penetrations, and seal them (which may result in de-rating your conductors). You also have to check on the rating and possibly spacing or proximity of NM boxes.
It could be that your source is confused about different types of construction (Types 1-5) or is aware of restrictions in some types of occupancies (places of assembly, e.g.).
Or PA is different.
Since I posted I started looking up some materials, looks like nonmettallic boxes are rated...I will just run everything by the inspector before we get in a bind.It has to be a rated box. No holes, normally drawn so there are no corner seams.