While these use contenteditable, it's not accurate to say they're just contenteditable plus some UI and interop.

None of these modern editors (Wordgard, ProseMirror, Lexical, Slate) use contenteditable for the document model. Rather, they have their own internal document model and use contenteditable as a kind of input layer where the editor monitors what the browser does, then translates that into actual edits.

Early editors like FCKEditor and TinyMCE were only wrappers around contenteditable. They used the DOM as the real document model, then intercepted certain keypresses and events and "fixed" the behavior when it wasn't correct (e.g. double enter inside a bullet list should switch to paragraph mode).

The result was rife with bugs and inconsistencies, and didn't allow for a proper split between the model and the view (e.g. to represent columns, video embeds, and so on).