open-ui is not proposing to make components look the same, it's proposing the exact opposite: to make the common components customizable enough that each design system / UI toolkit can use them rather than rolling their own.
Honestly, that just sounds like re-implementation with extra steps and an extra layer of wrapper/bridge nonsense. If I am going to completely redesign and customize the provided controls, I might as well roll my own. It would be way quicker and easy to maintain than a mountain of hacks on top of each other.