From what I can find, cryogenic treatment of copper changes the crystalline structure and is often used as a pretreatment for electrical components used in cryogenic conditions such as space.
It seems a bit pointless on instrument cables since the constant flexing will have an effect on the crystalline structure and probably end up somewhere near it's pre treatment state.
I suspect the main purpose of cryogenic treatment of instrument cables would be to extract money from gullible people. Give them a standard cable, tell them it's treateates and then listen to them waffle on about the incredible detail they can hear with it.