I agree that learning UI is one brick in the foundation and it can be learned on its own. But learning UI separately from actual product design is like learning how to season food without knowing how to cook it. You can do it all at the same time by building an actual product. When I design a side project I’m advancing my UI skills in tandem with other skills. It doesn’t make sense, in my opinion, to learn each individually and then try to bring them all together when they can be learned together.