Those machines are astoundingly aerodynamic: The rider's more reclined and closer to the ground compared to a typical low-racer recumbent. On the odd occasion I've ended up in the same race as the BHPC's faster handcyclists, I've found that drafting them barely has an effect. (Lack of important things to catch mean they can draft each other *really* close, inna pedal car style, which is impressive to watch.)
I think the different approaches to wheels reflect the handful of manufacturers of performance handcycles (Carbonbike seem well-represented here). There seems to be more of a sports wheelchair influence than recumbent trike, which makes sense. I'd assume that discs are more readily available in that world, as they're useful for preventing finger-entanglement, even if you're not moving fast enough for aerodynamics to come into play.
