Strictly speaking that doesn't say it includes one, just that there's room for one. But I hope you're right. I've looked at the PDF instructions and still wasn't any wiser.

it does not come with a PI, it's just plastic box, cables, HDD, and screws. (and maybe an SD card with their distro on it)

yeah, that is what it is. The price is lower than the separate USB3 hard drive kit from WD so it is hard to argue it is expensive, though.

The OS is put together by Canonical, until now it was based on Ubuntu 16.04 and they'll soon update it to Snappy Ubuntu Core so you get a fully 0-maintenance solution.

The disk isn't cheaper than in retail, if you don't consider the case to be worth a lot more than is realistic.