I just realize that rule #3, flipping the stone to mark as dead, still existed as a practice in modern scoring, makes dead stones easier to spot, and Chinese stones have traditionally been uneven (one convex).
It might be as a relic for this kind of “no removal” rule, not just it is more stable on one side. And it would make a lot of sense to use flat wood pieces as “stones” and can easily be painted with not just black and white surface colors, but also other colors for their flipped sides.