Yes - Cygwin is essentially the GNU tools recompiled as Win32 apps using a helper library for shared code etc.

Windows Subsystem for Linux (WSL) which underpins Ubuntu on Windows is new Windows kernel infrastructure that exposes a LINUX-compatible syscall API layer to userland and a loader that binds the two.

This means you can run real, native, unmodified Linux command-line tools directly on Windows.

So it's cygwin + ELF support in the kernel. I guess ability to run GNU tools + supporting ELF is being called "Linux". The latter piece seems pretty minor, especially since most things that run on ubuntu are open-source anyway.