I’m not a historian but I think FCKEditor and TinyMCE predated contenteditable. These were solutions wrapping textarea back in the day if my memory serves me.

My memory is fuzzy here, so I may be wrong. But I remember the generation of buggy WYSIWYG editors that preceded ProseMirror, and they were all contenteditable-based. They would typically start by attaching to a textarea, but then replace it with a div.