Are you sure that you aren't misreading that and are looking at fiberglass drums. My quick search didn't find anything with fiberglass coated wood. Siam Oak/ rubber wood with modern glues, especially multi-ply, wouldn't need to be fiberglassed. 
 A little bit about the fiberglassed Gon Bops 
here.The fiberglass on my Gon Bops aren't all that thick. Looks to be a single layer of fabric with gel coat.