It would make sense to me that a lifter kite with openings like this, or with multiple different airfoils, would be more stable. I don’t know the reasoning though. I’m guessing for this pill from the video, it’s that what’s causing the instability is vortex shedding, which is going to be chaotic coming from a symmetric body. Or that more air goes through the pill in the pill with holes, allowing less air and less force to go around the pill, giving it less force to move the pill.
Or from the comments: