To me it seems the point is to have an open engine to be able to create that kind of games; instead of starting completely from scratch, they wanted to emulate a base that is known to be working and actually fun to use, so the moment they reach feature-parity they'll have a solid foundation. After that anyone can create on top of the existing and make it even better.

This feels like people who are passionate about the base game and would like to see it being open-sourced but they don't own the source code, so they do the equivalent