I believe I didn't read anything that said GK104 has no ECC.
nVidia themselves says that GK104 has no on chip ECC. Right in this paper:
http://www.nvidia.com/content/tesla/pdf/NV_DS_TeslaK_Family_May_2012_LR.pdf
Here's the relevant part:
External DRAM is ECC protected in Tesla K10 and
both external and internal memories are ECC protected in Tesla K20.