Google interviews are four hours long and they don't tell you how did after the interview. There's no way he can possibly know the reason he got rejected.
The statement seems to imply that he would have gotten offered if he had been able to answer whatever this question was. There's no way he can know that.
It seems more likely that he was shaky in his other interviews which all added up to him being rejected. If you're an interviewer and the interviewee is doing poorly, do you tell them they're failing? I certainly don't.
