I think you misread that page. I think they mean that each SoC has one memory slot, which supports max 4GB RAM. So that 16 gigs is total amount for all the four CPUs. This still strongly points towards a 32-bit design.
Support for 16GB DDR3/3L memory via four (4) mini-DIMM sockets – one socket dedicated per SoC supports up to 4GB of ECC memory (supplied separately)
Which really doesn't sound as if the memory is shared, to me.