You were always qualified and they shouldn’t have rejected you the first time. This is an example of what not to do on the hiring managers part.
It’s entirely possible that the candidate was just off their game that day, or they weren’t “warmed up”. I get noticeably better at interviewing after the first few. It happens, but the interviewer has the information they got, and it’s not possible to totally control for that kind of variance.
What hiring managers should be actively minimizing, that most hiring managers I’ve seen do not, is variance that their interview process introduces by, for example, not standardizing and training their interviewers.
If the interviewer says "I think you're a good engineer", the next sentence should be "we would like to make you an offer".
If I could hire people by reading their mind and understanding their ability, without requiring an ounce of preparation, I would do that.