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.
contenteditable I believe was in IE 5.5 in ~2000, I remember playing with it back then. It landed about the same time as "Outlook Web Access" which is arguably the first SPA, originator of AJAX, and first use of XMLHttpRequest. I think TinyMCE and FCKEditor were a little later.
FCKEditor and TinyMCE were using contenteditable.
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.