You can be an interior designer without knowing how to make furniture.
You can also be an excellent craftsman and make really nice furniture, and have no idea where it would go.
So sure, UX coders, could make really nice buttons.
But if you have UX coders all going in different directions, and buttons, text boxes, etc.. are all different, then it is bad design, jarring, even if each one is nice.
Then the designer is one that can give the direction, but not know how to code each piece.