It’s true that every application is “considered” for admission, but a much smaller number are “seriously considered” beyond a first reading that lasts on the order of several minutes. Those are the applicants who might get interviews.
It’s true that the interview report is one of several pieces of the application. If a candidate is under serious consideration, the interview can make a difference, and often much more than a snowflake’s difference. If the candidate is not under serious consideration, the interview will not matter at all, and the weight will be less than that of a snowflake, even less than a hydrogen atom. It will be zero.
Interviewers are not “just” trying to see if a candidate is a “weirdo,” they are evaluating personal characteristics, including the depth and commitment to extracurriculars and intellectual spark.
It is incorrect to state interviewers are not “trained” in any way. I’m not sure where that rumor comes from. Some interview committees have extensive training, and many have some training. Others offer little to none, but the interviewers read and refer to the interviewer handbook.