Unfortunately, it comes installed with Windows. edit: of course it runs Windows. It also comes pre-installed with it.
Most people that I showed the Surface to were impressed by what they saw, but almost always came back to the same thing: it doesn’t run macOS.
I’m increasingly frustrated by this because it’s almost always a misunderstanding of how far Windows 10 has come, and how compatible the workflow now is regardless of what you might do for a job. I suspect most people haven't touched Windows since years ago and it's time for a serious second look if so.
I’ve written about web development on Windows 10 before, but even since a year ago it’s come such a long way. The Bash on Windows environment is now so performant and well supported that you won’t even know the difference if you move from a MacBook, and Microsoft continues to improve it at an impressive clip.
Unless you’re using Xcode, your workflow will almost certainly move from macOS to Windows as a non-event: it’s that good now, and I spend most of my time living in the Bash environment without any problems — even with complicated situations like Symlinks inside Bash now actually working fully.