So,
I love the writeup and the product but I am surprised that you didn't run into draft when doing your research:
It's exactly the competitor you were looking for and Nate is a totally awesome guy who I am sure you could even ask for advice while building your product!
Doing some research on my own, you focused on "GitHub for writers" which uses a very specific player in the version control space. Seems that being more generic in your search terminology with "version control for writers" leads directly to what you were searching for.