The boy didn't get punished. He got a verbal correction.

Honestly, the other adults shouldn't have had to reinforce the point to not hit girls.

Unfortunately, they probably felt they had to because the OP, going full on "boy mom" was excusing her sons behavior, publicly, as justified and demanding the girl he made to apologize too. She's diverting the spotlight from her son to the little girl.

She's also overstepping her parental authority by making a scene, demanding the children be given the same discipline. But she is NOT that little girl's parent and it is for the little girl's actual parents to decide how to handle her discipline. Not her, the OP.

She's interfering with their ability to parent their daughter. This "boy mom" energy is the reason why the little girl seemingly "got away with it". We don't actually know that, that's just her perception because she being driven by ego.