A very good explanation, but I believe you are missing some steps at the end. After he had the rover and bright object showing up white and the sand showing up

A very good explanation, but I believe you are missing some steps at the end. After he had the rover and bright object showing up white and the sand showing up as black, he removed any large connected white objects (In any image the largest block of 'non sand' would always be the rover). He then drew a circle around anything left over, which in this case was the mystery bright object. The beauty of his approach is that its generic and can be applied to any photo of sand and rover to find interesting anomalies.