That article covers a bug in implementations of blurring in browsers. It shows how when a 200px red circle is fully covered by a blur with radius 16px, today you will see a 232px sized blurred circle. But when the circle is shifted down from under the blur, so there is one pixel of space separating the two, you would still expect to see 15 px of blurred red circle. Instead it goes 18, 17, 16, 0, 0, for every successive frame. That’s a bug.

It’s not an implementation bug, it’s a deliberate design limitation in the name of performance. (There are quite a few practical limitations on backdrops in the name of performance.)

It’s not impossible that this will be adjusted in the future in some way, like how browsers quite some years ago shifted to premultiplying colours so that a gradient from white to transparent doesn’t go through semitransparent grey, but browsers are acting according to spec.

It’s an implementation limit of a limited backing layer size. So no, it’s not a bug. But neither solution correctly blurs objects that insect with the layer.