"Are carbon fiber speaker cabinets inherently better than wood or metal ones"
The answer is no. All materials have specific characteristics and need to be used correctly and in the right application. No material is inherently superior across the board, including aluminum or wood.
Actually Rockport used cast aluminum enclosure sections which is very different than machined cabinets. Magico used extruded and machined section. Their designs have more to do with the implementation than the materials.
We just concluded a year long evaluation of different materials, including cast aluminum and carbon fiber, for our Gen 2 Apollo speakers. For our speakers design we found that a fiber reinforced molded custom engineered polymer had the best characteristics. Cost is between aluminum and composite, aluminum being the cheapest.