You have it correct, this UI DLC won't show up on the DLC startup menu because that list is hardcoded in the game engine (or queries a DLC tag which I have not discovered).
The side bar / ribbon is only displayed when options - interface options - "single player score list" is UNchecked (so the two don't overlap, and it also gives a convenient way to disable it).
This UI DLC affects base game UI, so it is not friendly with other mods which do the same. But it's modular, so you can try disabling conficting modules by removing the corresponding subfolder from "UI_bc1".