Doesn't it come with `pacman` too?
MSYS2 is there to just provide the basics so you can develop programs that are Windows native but use some of the tools that have really strong Unix dependence like shells or Make. They depend on the existence of syscalls like `fork` or forward slash being the directory seperator.
Without Cygwin enabling the path, it wouldn't be possible to build GCC for Windows without completely changing its build system. It would be a DOA fork while Mingw and PE32+ support is a part of GCC nowadays.
The nice and genius part of MSYS2 is that it is there to primarily encourage you to develop native Windows software that has better cross-platform behavior rather than Cygwin alone. If Microsoft made a better, free of charge C compiler in early 2000s that is adhering to the standards better, we wouldn't probably need Mingw to build cross-platform apps. Now MSVC is still free of charge for only open source and individuals.
[1] "Cygwin POSIX emulation engine", https://packages.msys2.org/base/msys2-runtime [2] https://github.com/msys2/MSYS2-packages/tree/master/msys2-ru...