It is written in JS and it looks like that JS can be edited in the editor and reloaded on the fly. That's the core of Emacs I think, being able to recode the editor on the fly.

I haven't actually tried that with YMacs so I don't know how well it would work. But I could see something like this forming the basis of an actual serious editor. The one thing I feel is missing from Emacs is a decent display engine. Of course that's not a new idea.

EDIT: I just fact checked my comment and it turns out that you can't just reload code into a running YMacs. If that were supported then I think it would be fair to call it an "Emacs like editor" or even just an "Emacs".