No dumb questions. Even if the bumpers were "just fiberglass" you would still polish and seal them. Fiberglass can be gelcoated which requires different prep in some regards than other surfaces, but also needs to be protected.
Most of today's cars use an integrated front and rear fascia which is made of a flexible urethane material. This is painted with the same basecoat/clearcoat system as the rest of the car (although possibly containing a plasticizer for flexibility) and should be treated the same as the rest of the car, although these fascias may build up heat faster during polising due to the lower thermal conductivity vs. metal.
