Yeah, you’re right that it doesn’t make sense if you think of the rectangle as a flat thing hovering above the background like a piece of paper.

But I think the intention is to simulate the rectangle being the top surface of a 3D box. If you shine a light at a cube on a desk, the shadows are very sharp right where the cube touches the desk, and then further away are softer.

I think that’s the effect they’re trying to achieve here?

So it’s not changing the gaussian for falloff, it’s changing the shape of the shadow, from a rectangle to a rhombus, and using different gaussians along the edges.

Here’s what it’s doing with the settings tweaked to make it more visible: https://imgur.com/a/NUXhHrn