The statement is true. By definition relief in sculpting refers to the elements that are placed on top of a flat surface. Things that are "low" only stand lightly out from the background making it seem as if they are part of the background while those in "high" are carved further from the background making them seem like they are on top of the background.
If you can, take a phone flashlight and shine it in. Another tip is if you have one of those ear things that light up and you can look into people's ears. Try shaking it around just slightly to see if you can hear it in there, if not it has fallen elsewhere.
