Or maybe better said is that death is total absence of *awareness* of motion.
Sensation would not be Motion alone, because Motion never stops regardless, but our awareness of motion can and does stop at death.
The connection between awareness of smooth motion and pleasure seems clear from the passages. Smooth particles will move more smoothly and lead to awareness of smoother motions due to their shapes while rough atoms the opposite.
Of course we are presumably talking about small bodies here rather than atoms directly.
But the bottom line is that awareness of smooth interactions through motion are what is being felt. The analogy of pleasure as including both stimulative and nonstimulative experiences would be that smooth motions are going on in the healthy living body whether the body is being stimulated in some way or whether the body is simply functioning in its regular healthy way.