It isn't supposed to be balls on her balls... For a good time, Zheng grew a bizarre habit of drawing futanari with incredibly saggy balls, and when they cum, the balls somewhat goes up, and what you see as the "balls on her balls" is actually the skin that holds those saggy balls. Its supposedly an exaggerated effect to show that the futanari's balls also have some action when they cum.
I'm not saying that what Zheng draws in their art makes any sense or is realistic at all (cause it's obviously not), and I think the flag is pretty justified.