That's because the system is fully 64bit... with a 256 bit wide bus. Each of the cores can have full access to a segment of memory.
from the conclusion page, and something Apple should consider.
It is still worth noting that if Apple were to release a Core 2 (Conroe/Kentsfield) based Mac similar to the Mac Pro, it could end up outperforming the Mac Pro by being able to use regular DDR2 memory. The Intel 5000X chipset and its FB-DIMM memory are really designed for multi-user server and high end workstation workloads, where large amounts of simultaneous reads/writes are able to mask much of the latency penalty of FB-DIMMs. The problem is that the Mac Pro is the only high end desktop Apple offers without an integrated display, so inevitably it's going to attract a lot of users whose usage models aren't best serviced by the Intel 5000X chipset.