I would like to add that interviewing requires quite different skills from engineering. A good tech interviewer has to be a good engineer, but a good engineer might not be a good interviewer. I have seen good engineers with poor ability to read people, with strong bias of "curse of knowledge". They are more likely to fall for saleperson-typed candidates.
Therefore, a hiring manager should have the ability to tell which engineers are good interviewers, and give more weights to their opinions. Pay extra attention to those interviewers who is more of a big talker and less of a listener, if you have to ask him/her to interview. If one can't be a good listener to his/her teammates, I can't imagine he/she would listen to and observe a stranger with any substantial effort. Unless he/she is a genius in reading people, his/her feedback can be mostly ignored.
Therefore, a hiring manager should have the ability to tell which engineers are good interviewers, and give more weights to their opinions. Pay extra attention to those interviewers who is more of a big talker and less of a listener, if you have to ask him/her to interview. If one can't be a good listener to his/her teammates, I can't imagine he/she would listen to and observe a stranger with any substantial effort. Unless he/she is a genius in reading people, his/her feedback can be mostly ignored.