"Spatial Finder" was still a hierarchical viewing of the filesystem, yes.

There are many articles on the subject, but the "spatial" was about the idea that files and folders "remembered" where you put them, so that the same file was always in the same (pixel-exact) space you chose to place it and folder windows remembered their exact same size, shape, and location on the screen. Regardless to how you navigated to them, but since on the Mac Classic OS the Desktop was the "root" folder, you typically navigated to your folders in the same physical way every time (to the point where you might muscle memorize where your files and folders "are" in the space layout of your screen).

As a Unix user it used to be you could get a basic spatial mode from Gnome's Nautilus, if you wanted to play with it, but it looks like Gnome File Manager dropped that feature somewhat recently.

Yeah, it's really weird how Windows and OS X have a 'loop' where the Desktop is a directory inside of your home directory.

OS9's file system was actually very similar to modern OS X (earlier incarnations of HFS). So it's an abstraction of a standard directory hierarchy, albeit a very elegant and disciplined one. The major difference under the hood was the presence of resource forks for various kinds of metadata and UI (icons, for example).