I believe it's a complete re-implementation. I think with so many 'deep' changes to the functionality, rewriting would probably be easier than forking.

The code statistics on github[1] show almost all C++, while vim is written in C. According to the author the project began as a reimplementation or merely a rewrite of vim then turned into a more ambitious reimagining of how to design a modal editor.

[1] https://github.com/mawww/kakoune

It is written from scratch in a very modern C++ style, I advise every C++ programmer to look at it. Also the code is quite well organized.

From the github repository: https://github.com/mawww/kakoune reply

    Vim inspired — Faster as in less keystrokes — Multiple selections — Orthogonal design

You can also use a cat or nothing as "helper".