In the comments of the latest blog post, it was mentioned that there is an OpenGL build for Windows, but there were problems with it. Assuming it is still being worked on, or there is enough time available to be able to continue to work on it, if those problems are solved (don't know if the issues are more with the game engine, or with OpenGL in Windows), that could maybe possibly help with the DirectX 11 requirement (don't know what version OpenGL support would be required).
Always wondered why games that support Linux/OSX obviously have working OpenGL support yet you can't use it on Windows. Probably just not worth the effort making it bulletproof when everyone has DX anyway.
In the late 90's it was more common to have multiple API support.
