[saidajigumi]

Can you have the skill to come up with a good design if you have no clue how to implement it or how someone else would?

This is a straw-man. Real-world designers absolutely do have a sense of implementation constraints for their target platforms, yet are usually not deep implementation domain experts. Likewise, experienced UI devs often have a sense of design, but are often not going to draft and refine a whole-cloth new design. I’ve also met a rare unicorn or two: those magic individuals who are fully capable developers and designers.

Put another way: UI design and UI development are adjacent specialties. Visually, they’d be partially overlapping regions in a Venn diagram. A UI designer or a UI dev who somehow knows nothing of their peer discipline will be hamstrung. I’ve met devs who fit this bill, not by their own design, and they’re usually radically unhappy with their work until they manage to pivot to non-UI development.