My phone rang during the interview. I silenced it and apologized to the interviewer but carried on. After the interview I listened to the voicemail that my old boss had left. “When we offered you the job I had you mixed up with someone else. We hired the wrong person.”

They called you and said that because they were being blamed for something on their end and it was going to make them feel better to say you should never have been hired. They didn’t make a mistake, they’re just trying to make themselves feel better. You did fine and you got a good learning experience while you were at it!