Both on the Nim website and Github readme the recommended Windows compiler is listed as MinGW. I downloaded the provided zip files from the Nim website but they are actually MinGW-W64, not MinGW. MinGW-W64 forked from MinGW over a decade ago to improve compatibly and provide the ability to compile 64 bit executables. The Github page insists "MinGW 4.3.0 (GCC 8.10) is the minimum recommended compiler", yet the MinGW-W64 version provided on the Nim website is GCC 6.3.0. The latest versions (GCC 8.1.0) can be found on their SourceForge readme as x86_64-win32-seh and i686-win32-sjlj.
Whoever added the information to begin with probably just abbreviated the name but seeing as they're two independent projects I think clarification (and credit) is due. I wasn't sure where to post this so I can open a Github issue if need be.