I cannot find anything at all that supports such a claim. Do you have any links or supporting spec sheets? All I can find are the same "Supports ECC: Yes" listing for Epyc, Threadripper, and Ryzen. The only spec differences I can find for Epyc vs. Threadripper on the memory side of things is 8-channel vs. 4-channel.
Look at specs for any X399 board and they will say something like "Supports Quad Channel DDR4 3600+(OC) & ECC UDIMM Memory" or "8 x DIMM, Max. 128GB, DDR4 3600(O.C.)... MHz ECC and non-ECC, Un-buffered Memory". Both omit buffered/registered DIMMs.
