As I recall, when the E3-12xx chips showed up on Ark for a few days by mistake, they clearly indicated ECC UDIMM support, but did not indicate ECC RDIMM support. What appears on the SuperMicro pages above is consistent with that.
The reason you need RDIMM after the DIMM reaches a certain size is because the CPUs memory controller cannot drive the memory cards correctly without the extra register to help manage it. Without the register, the CPU memory controller has to deal directly with all of the memory chips on the DIMM. Apparently, Intel's designs for the E3-12xx series can handle bigger memory chips directly - thus support for 8GB UDIMMs.
Having done this, they also apparently dropped the logic required to support RDIMMs at all.
Now we have a marketplace problem. None of the memory manufacturers have 8GB ECC UDIMMs because - until now - no CPU or chipset existed that could use them. No need to manufacture a product that has no demand...
Time will fix this. Once the E3-12xx chips start hitting the marketplace in a big way you'll see the 8GB ECC UDIMMs show up too. Should also note that the UDIMMs have to be dual-rank. Apparently this chip will not support quad-rank DIMMs at all, so expect these 8GB ECC UDIMMs to be on the expensive side, at least for a while.