That over-simplifies things a bit much. Hanging a bit over the hole does not mean they would fall if not jammed together. One ball should be more than 50% over the opening for it to be "in" and having both doped in because "some edge" is over the whole would change games. They need to define what "leaning over the edge of the slate" means, is it any part of the ball, more than half? I mean a ball may be leaning over the edge if it's about a sliver over depending on how you define "lean".
Both balls should be hanging over in air when jammed for both balls, or even one to be considered "in" unless the rule makers think it's OK to drop one in and then re-position the other ball where it was sitting before.
The WPA rules state that if the ball "would" drop (not do actually do it) if the other was removed then it's considered pocketed. It does not just state that if the edge of the balls are over the lip but they are jammed that they are both in.