I thought they made themself pretty clear in this post. Not to be rude, but you seem to be searching for a reason to be confused. They are rendering an animation with a transparent background. There are some frames of the animation in which there are no objects in view, so the frame is 100% transparent background. Render times are usually subjective, but they are directly comparing the render time of frames with objects in them to ones that are completely transparent. They are wondering why Cycles can’t render a completely transparent frame very quickly, and I think Dolores gave an excellent response.
