If people look at 9-10secs in the vid. You can see the movement of his balls. You can see his scrotum (the dark round area by under his hand), that’s one ball. Then to the right you see another light skin bulge in his pelvis/pubic region. That’s his second ball up inside. It moves in sync as he strokes.
His balls aren’t sagging. They’re tight.
I’ve also seen a few guys get hard and one ball goes up as the ball sack tightens. I’m sure people can find examples in porn that shows what we’re talking about.