"I don't parent to have a well-behaved child, I parent to have a well-behaved adult."...
I think all of us have certain behaviors about which we are simply inflexible. Running toward a busy roadway is one which comes to mind. Even the most gentle of us will shriek, raise our voices, grab our child, scold, weep and admonish our toddler if we see her take off headed toward traffic. It is not acceptable, and we take pains to make that clear to the child. And in general, it works for us to do that. In fact, it works much better for gentle parents because our children are so unaccustomed to seeing us react with stern emotions and rigid guidelines. We don't give dd a lot of rules to follow, and the few that we do, she pays attention to.
Some of us choose to treat hitting with a similar approach. We teach that: HUH UH, that is NOT acceptable, you will NOT do that. I can't speak for other parents but we have never punished dd to make these points. We simply communicate honestly.
Pretty much that's what this other mother did. She didn't threaten violence. She spoke honestly. She reacted honestly. I'm sick and tired of your meaness. It's wrong. If you were my child I would spank you. It was a good, honest encounter that was proportional to the problem. They way the boy's reaction was described, it seemed to be a lesson that was sinking in and making an impression on him. Now that he sees his mother negating the other mother's message, that effect is probably lost.
Maybe some of you mothers are nonchalant about hitting and don't think seeing your toddler smacked by a four year old boy is a big deal. Maybe that's why your kids are still hitting.