One of the selling points of neovim was to make it more easily embedded into other programs.. doesn’t seem like anyones trying to do so from a quick googling though; maybe one day there can be true unification of the holy editors
1. Language is more extensible because of macros so dealing with editor state can be hidden.
2. Language is more dynamic with advice and eval-after-load and such is extensions can play well together
3. For all it’s difficulties, having buffer-local variables and dynamic scope seems to make writing extensions easier
As far as I can tell, making Emacs cooperate with neovim would require shoehorning the editor model of one editor into that of the other, likely causing many extensions and expected behaviours to break.
There is, I think, a difference between making a new application that can use neovim for editing some bits and making an old (or, indeed, very old) application switch to using neovim for editing.