There's nothing wrong with using carbon fiber for a compressive load like this. Like most engineering the devil is in the details - how do you interface the carbon to the metal end caps for example? They likely are a weak point as the compressibility will be different for both materials making them want to shift (or fracture if they are glued together) during pressure cycles. I'd love to know how they solved this (or if they didn't and that's why we're reading this).