Um, I might just be stupid I guess. I saw "if you're not using Omnigraffle 6" and took that to mean "If you're not using omnigraffle at all", so then I expected it to be a folder of assets. So if I open the zip file, there's a single folder with 4 images and a plist. I assume that on a mac the .gstencil would open in Omnigraffle, whereas in windows it just opens as a folder.
Anyhow, just operator error I'm sure.