Foam rubber is indeed a foamed plastic rubber. And the paint is a plastic, and the vinyl upholstery is plastic, and basically, everything on these cars that isn't metal is plastic or glass. Fun stuff, the world of materials engineering.
And knowing fiberglass is a plastic can be useful. There are properties to keep in mind about it. Paint adhesion, heat effects, chemical resistance, et cetera.
Practically speaking, the fiberglass pieces of an S12 come across as plastic, because they're more flexible than a typical GRP part, and are made of a greater ratio of plastic-to-glass than most GRP parts. They gouge like plastic, warp in heat like plastic, and adhere to paint like plastic. S12 parts are plastic, with glass fiber in them. Boats are glass fiber with plastic on it.