Flask-AppBuilder is opinionated fork of the Flask-Admin. It follows similar conventions and uses some of the Flask-Admin code, but comes up with non-default skin and a little more batteries (like user management interface out of the box). If compared directly, existing Flask-Admin components are much more featureful that their counterparts in the Flask-AppBuilder.

Flask-AppBuilder API is not compatible with vanilla Flask-Admin APIs, so you won't be able to exchange components between them. And sure, you can build something similar to Flask-AppBuilder on top of Flask-Admin without changing its core. Or maybe someone already built it (like Flask app skeleton with built-in administrative interface).

I explained reasoning behind default Bootstrap skin and lack of built-in user management in the blog post.

I have nothing against forks, it is good to have multiple options to choose from.