Mac ships with their own posix subsystem, WSL allows you to install a bunch of popular flavours of linux on top of Windows. I personally run Arch Linux and have access to a lot more bleeding edge packages.
To clarify, Mac ships with a native Unix [1], derived from BSD, not a posix subsystem. It has its own init (launchd) and its own GUI, but most of it is a Unix.
Using Macports or homebrew gets you most of the packages that are available for the various Linuxes, GNU utilities etc.
But what are you doing in WSL that you can't do on a Mac (or, of course, a Linux machine) without a 'subsystem'?
Sincere question, because the 'Mac is not Linux' things that wind me up tend to require X11 to fix, WSL would be no different.