It must be design differences and how much material they use in each one. The density of aluminum is indeed higher than that of carbon fiber. About 2.6 gram/cc for aluminum and about 1.75 for carbon fiber. So the carbon fiber shaft should be lighter if they both had exactly the same dimensions. I guess they must be using more carbon fiber (or more something else) in their driveshaft as compared with the aluminum version.
On the surface, it sounds a little goofy because carbon fiber is also a lot stronger (tensile strength at least) than aluminum or even steel.
So, you got me. I'm not an engineer, but I guess the BMR boys (who undoubtedly are engineers) are aiming for a stronger driveshaft.
One more consideration is the cost of $1100 versus the possible gain (maybe 15 HP) put it at $73 per HP. The $600 ones come out to be about $40 per HP.