I’ve been thinking about this question and the others related to it that Josepha Haden Chomphosy posted last week. There’s been a strong comment discussion so far, and I discovered what my view on it is now by noticing what is going on in the grammar of “Editing” versus “Editor.”
“Full Site Editing” ends with a gerund, which is a verb that’s used like a noun, but even more specifically: “a gerund names or identifies actions, states of being, and states of mind rather than people, places, things, and ideas.”
A gerund isn’t a verb showing what someone or something is doing; it is the thing itself that’s doing something or being something or having something done to it.
“The … Editor” is a noun describing a thing that is also a place. You can “go into the editor.” But you “go into editing mode” — a change of state, not a different place. You acquire different capabilities.
Maybe eventually full site editing mode in some WordPress themes will mean you can edit layout (presentation) and content comprehensively. Maybe there will still need to be an option to zero in and focus on one piece of content. Currently, these are two distinct things that feel very different even in FSE themes. Complicating matters, classic, block, and hybrid themes without FSE are totally different user experience contexts.
