He believes the best approach is to create the new editor within the Customize API, giving it live previews from the start.

I think that Nick isn't saying we need a new editor but rather a new editor experience. I interpret this to mean that I think that the more we can treat “the Editor” as a component rather than the admin screen the better. We should be able to instantiate a TinyMCE editor anywhere to manipulate content and content blocks, whether this be on the admin screen or in the customizer. For example since Shortcake integrates with the TinyMCE editor then the Post Elements interfaces it provides then (mostly) automatically become available to the TinyMCE editor embedded in the customizer via the Customize Posts plugin.

So the Editor component should be appear on the admin screen and on the frontend (with customizer bootstrapped). When you do “Preview” from the edit post screen this should take you to the frontend with the customizer bootstrapped so that not only can you preview the changes you made on the edit post screen but you can also continue to edit changes in that context and see the additional changes live. This is implemented today in Customize Posts.