I don't think bash itself has been ported, it's simply running under a Linux subsystem. I imagine it's similar to the discontinued POSIX and UNIX[2] subsystems. In this case, bash probably believes that it's running in on Linux, not that it's been ported over to compile in Windows.
[1] https://www.wikiwand.com/en/Microsoft_POSIX_subsystem
[2] https://www.wikiwand.com/en/Windows_Services_for_UNIX