I liked the ideas and especially the paperclip :)

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

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