Thats fair. Could you have the same iterative experience with an LLM, but starting with a prebuilt base and iterating from there?

Yes. Even without Claude design and just Claude code, it can use existing design and build out new mockups in-app, which is much easier to demo , tweak and then implement the backend (if any) - all through Claude Code (or Codex if you prefer that). We use both and are now leaning more towards Codex over Claude