"Fade out" when discussing music, is a term that dates back a LONG time from the first days of the radio. It's when the volume of the recording just decreases while the instruments continue at full force. That's what a fade-out is.
Also, the instruments in Rime come to a complete stop. There's no question the guys have stopped playing and Bruce has stopped singing. On a real "fade out," it sound like the guys went on playing, even if you can't hear it anymore...