You approach a very broad subject, and are doing it sideways.
There's 3 factors to ram speeds. The first is cpu memory controller, it operates at a specific frequency, for the threadripper it's DDR4 2667 (officially). Second, there's the motherboard, which depending on model can take anywhere from 2133 to 4000+. Third is the ram. DDR4 stock default speed is 2133. That's as low as it goes. But can be raised upto 2666 by the cpu choice, without any ram OC. Speeds beyond 2666 are all considered OC, and are usually gotten by a simple enable of the XMP profile.
Amd doesn't use the same hyperthreading as Intel. They use multiple dies under the lid. Each of those core dies is connected by amds infinity fabric which operates according to ram speeds. This makes ram speed somewhat important, you get better cpu performance with faster ram. You can get upto 20ish% performance increase just moving from 2133 to 3200.
Some cpus/motherboard/ram combos have had issues actually getting 3000 or higher, and maintain stability, 2933 was the best they could do.
None of which has anything to do with cpu OC or anything other than instituting XMP profiles already included in the ram speed.