Some time ago Microsoft announced a compatibility pack to get OpenGL and OpenCL running even on computers whose hardware does not provide native OpenGL drivers. It is basically OpenGL-over-Direct3D. Or that is at least my understanding of it, hopefully this description is sufficiently accurate to not cause audible groans on the devs who actually know what it is doing under the covers. More actual details can be found in this blog post.
An OpenGL implementation is a whole lot of work and writing one from scratch is a multi-year project. Instead of doing that, Microsoft chose the sensible approach of taking the Mesa implementation and porting it to work on Windows. Typically large corporations do this by the vendoring approach, that is, copying the source code inside their own repos, rewriting the build system and treating it as if it was their own code.
The blog post does not say it, but in this case that approach was not taken. Instead all work was done in upstream Mesa and the end products are built with the same Meson build files . This also goes for the final release that is available in Windows Store. This is a fairly big milestone for the Meson project as it is now provably mature enough that major players like Microsoft are willing to use it to build and ship end user products.
 There may, of course, be some internal patches we don't know about.