The basic UI design for the editor is the same, as you’ve  noted, but the UI is all new code.

We reuse basic parts, esp server side where we built on the same foundations as we use in all my recent products, Drummer, FeedLand.

But this is a very new way of developing, I have a great development partner — Claude Code, which has made it possible to try out lots of options, and extensive iteration, far more than a human developer could do.