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This is a CMS isn't it? Haven't we been doing this forever? CMS for apps has been a bit behind where it is for the web, but it still exists.

A CMS (e.g. Contentful) provides the content/data. SDUI is about using that same technique for the layout of the user interface itself.

You can still easily allow users to set layout from a CMS. It's been a core feature of platforms like AEM for over a decade.