Usually headless means that serving the content is done only through an API while creating content (and -types) is done using an admin interface.

If you look at the docs of Strapi you see that this is the case for Strapi as well.

You would train editors on... editing. It does have an interface so people would need to learn how to use it.

True. Depending on the deployment scenario there could be some additional factors. For example, take a static site that runs on netlify. There is a hook on headless CMS save that triggers a build. The build is usually quick, but its not instant (like Wordpress for example).

The UI?