I assume that the limitations of OSX's actual window manager is the reason that a proper tiling WM like those on X11 are practically "impossible". This might be the next closest definition?

I could see this behavoir "if you move a window to the left/right and one already occupies that space, both are resized to fit". What happens exactly?

I am not sure what you are asking. If you "could see" the behavior what is it that you do not understand about what happens?

Not trying to be a nitpicky jerk, genuinely interested in answering your question if I can.

Sorry. I meant "I couldn't see".

When I move a second window to left-half of the screen with Cmd+opt+Left, it just resizes and moves there on top of the previous window, the 2 windows aren't tiled one at the top, one at the bottom of the left-half of the screen. I assumed that's what it should do.

Maybe you downloaded from spectacleapp.com rather than https://github.com/fjolnir/spectacle/downloads which is the binary for this fork.

This is embarrassing. Yes, that's that I did. Thanks :)