Looks like, if a posix system is detected at compile time, it will return whatever it gets from the `uname` system call in `sys/utsname.h`:

https://github.com/nodejs/node/blob/master/src/node_os.cc#L5...

Microsoft probably hard-codes a response of "Linux" (or whatever would be normal for Ubuntu) for that call to prevent Ubuntu binaries from freaking out.