The AO didn’t guarantee acceptance, you are reading too much into the post. I think @Kristin_White has good advice. There are good reasons from the school’s perspective to be encouraging. One, if they accept the kid, they want the family’s interactions with the school to be positive and therefore have the family kindly disposed towards the school. And second, it is just kind. There’s no reason a 13 year old needs to walk out of an interview thinking it was a disaster, or they bombed. Or that the AO made it clear they had no hope at that school.
That’s where @Kristin_White’Kristin_White’s advice comes in – the adults need to put the experience in perspective.