generally speaking, in this industry if they dont list it as an alternative material, its fiberglass. mfg's love to spout out about their kits/spoilers/etc being carbon fiber, polyurethane, or even if they are using the newer flexible fiberglass resin so it reacts more like poly.
my money is on it being fiberglass. which means heavy, and easily destroyed. as well as more difficult to work with as far as the installation goes.