Wow, that's really weird. On the HTML page, they all say 2133 MHz. It's only on the PDF datasheets that they say 2133, 2400, and 2666 MHz respectively.
So if you hadn't mentioned the datasheets, I wouldn't have even bothered to look. If I were shopping for Ryzen with ECC, I would have just looked at the HTML page, decided it wasn't remotely what I wanted, and just moved on.
I'd be interested to hear about your experience. I just built a Ryzen with ECC, but I went with Kingston RAM. Everyone was telling me that Samsung was the best, but I thought all the Samsung ECC was 2133 MHz because I was just looking at the HTML pages and not the datasheets. People told me it would overclock fine, but I preferred to get something that was factory spec'ed as high as possible. All other things equal, I'd prefer factory over overclock for the same reason I want ECC in the first place.
So in the end, I went with Kingston KSM26ED8/16ME (16GB 2666MHz DDR4 ECC CL19 DIMM 2Rx8 Micron E).
Product page: https://shop.kingston.com/partsinfo.aspx?ktcpartno=KSM26ED8/16ME
And that's worked fine for me. But I'd be interested to hear your experience with the 2666 MHz Samsung ECC.