It's non-lethal in the same way pellets or even smaller calibers are non-lethal. Just look at this:

This is the caliber needed to kill the animal with a single bullet at center mass. For .223 LR, you can reliably kill rats at center mass but can't reliably kill squirrels at center mass. That's fucking it. It doesn't mean squirrels are somehow impervious to .223 LR or that you can't kill a squirrel by aiming at its head or that a squirrel can survive getting mag dumped or the squirrel would survive getting shot by a .223 LR at less than 5 yards. It just means if a rat gets hit by a .223 LR and it wasn't at its tail, the rat is completely fucked while a squirrel may or may not immediately die.

Going back to rubber bullets, I'm pretty fucking sure they're good enough to reliably kill rats. And those "uh aktually, you're suppose to aim for the ground and let it bounce up to hit people instead of aiming at people directly" rounds are good enough for squirrels and rabbits.