Yes it was a real slap. It was the perfect reaction from a real man and a gangster who a woman had just told she aborted his baby. Corleone family as shown through out the whole trilogy is quite devoted towards family traditions, the slap was out of desperation.
There is no no quotes from Keaton on whether it was authentic but this passage from Deborah C. Micthell’s biography “Diane Keaton: Artist And Icon” is pretty noteworthy: “When her parents saw Godfather II in Los Angeles, the audience applauded when Michael slapped Kay in the confrontation scene. She (Keaton) defensively explained: ‘he was a horrible character…I say to Hell with those people who applaud. My parents were with me.
This kind of reaction can only come from someone who really experienced it. I hope it answers your question.