It's not just "literally use Neovim as the editing engine", I've tried to make editing the text in "Sublime Mode" work too, including running Sublime plugins. Work in progress, but afaik the most complete Vim integration for an existing editor. Waiting on updates to both Neovim and Sublime Text to polish it a bit more.

VSCodeVim is using it as a reference for some of their new work as well.