I can't say what others parents do with their own kids.Smacking is not for me.
If someone threatened my child , said they would smack them, they would no longer be a friend of mine , i would not take ay excuse .For me that is not on,there is plenty of ways to help kids play together, they are learning how to play how to socialise .

How the heck is thretening them with a smack teaching them not to be rough?