This action is neither stropping, or sharpening.
When you strop a straight razor on a piece of leather, you are giving the treatment to both sides of the blade. Similarly sharpening requires you to repeat the action to both sides of the edge. A cartridge locks one side of the edge into the housing, so sharpening, or stropping cannot occur, as you need access to both sides of the edge, and one of them is inaccessible.
At best this rubbing of the edge on a piece of denim is cleaning dirt, and debris from one side of the blade edge: fairly ineffective, and I would expect any improvement to be very minimal.
When you strop a straight razor on a piece of leather, you are giving the treatment to both sides of the blade. Similarly sharpening requires you to repeat the action to both sides of the edge. A cartridge locks one side of the edge into the housing, so sharpening, or stropping cannot occur, as you need access to both sides of the edge, and one of them is inaccessible.
At best this rubbing of the edge on a piece of denim is cleaning dirt, and debris from one side of the blade edge: fairly ineffective, and I would expect any improvement to be very minimal.