The point of the Windows Subsystem for Linux is that you don't need to run a VM. It is much more efficient in terms of speed (you're not keeping a whole virtualized Linux kernel and everything else running) but also in terms of energy. I can run bash on my Surface (not pro) 3 without any noticeable hit to battery life and without hurting performance.
If 'real linux' is the Linux userspace, then that's what you're getting with Ubuntu - all of 'Linux' except the Linux kernel. WSL allows the NT kernel to handle all of the Linux syscalls. If you consider just the public interface, then WSL is also 'real linux'.
Of course this is all theoretical because there are still some bugs here and there. Those are being dealt with and WSL is making great progress.
If 'real linux' is the Linux userspace, then that's what you're getting with Ubuntu - all of 'Linux' except the Linux kernel. WSL allows the NT kernel to handle all of the Linux syscalls. If you consider just the public interface, then WSL is also 'real linux'.
Of course this is all theoretical because there are still some bugs here and there. Those are being dealt with and WSL is making great progress.