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.
I couldn't find whether it is completely written from scratch or a vim fork? If not, why is it not a vim fork?