My mother in law thought it was ok to smack my kids hands and to tell them off.
It happened like this:
My son wasn't 2 yet. He tried to get something from a small table. I said no to him and pushed his hand back and again.
Then mother in law came and just smacked his hand and yelled NO to him.
She only did this one time ;)
She tried it again when we were with the family in a restaurant. My son didn't like to wait for food to arrive and started to let us know he had enough. He wasn't even crying yet, just getting bored.
She to my father in law who was sitting next to my son:
Give him a smack on his hands for crying.
He looked at me and I looked at my father in law with eyes that were clearly saying:
Dare to touch my child!
He's a nice man and we never had any problem with each other. He totally understood me and did nothing.
So, no you're not overreacting!
I think in only extreme or special situations it's ok for someone else to tell your child off. But anything physical, shouting or swearing is absolutely a big NO NO!!!
I think you could even report it as child abuse. No one is allowed to smack, hit or even touch your child in any way that could be harmful to your child.
It's understandable they were not happy with something being thrown at their dog. But your child is so young and unable to understand that. They could have let you know that they were not happy about it. Maybe said something to your child like:
Please don't throw something to our dog. He doesn't like that.
But smacking.... NO NO NO NO NO NO NO and again NO WAY THAT WAS OK.
It's totally normal for you to be angry about it.