This jsfiddle demonstrates it a bit more clearly.

http://jsfiddle.net/liamnewmarch/GgUV2/

The top gradient is from transparent to red, the bottom is from rgba(255, 0, 0, 0) to red.

Notice how the top gradient fades through grey - this is because 'transparent' is a synonym for rgba(0, 0, 0, 0).

What the parent article picks up on, is how this is visible via Firefox's antialiasing.